In the beginning of the 80's The Thing, we see a spaceship crash land on earth 100,000 years ago, and it freezes in Antarctica. The creature within can assume the forms of its victims. The ability for advanced mimicry is its way of hunting and hiding, but a creature without a natural form would not be able to construct a spaceship, and if it is a super predator then it would be content on its own planet and not be able to conceive of the idea of interstellar hunting. How did it get a spaceship to come to Earth? A canon answer if possible.
There are a couple of seemingly incorrect assumptions in the question (but that doesn't make it a bad question):
a creature without a natural form would not be able to construct a spaceship
A creature that can morph its body into whatever shape it wants (and we have amble evidence that it is not restricted to the form of its "hosts"... as witness by the severed head sprouting legs and crawling off, or the animated blood, or the long-legged thing that sprouts out of the dogs and climbs into the ceiling) could almost certainly create any sort of manipulative appendage it wants to.
It can take human form, with all the manual dexterity that a human has, and a human (well, human*s*) can certainly construct a spaceship. Why would we think that it would be incapable of such precision prior to taking human hosts?
and if it is a super predator then it would be content on its own planet
Why? Assuming that the biomass on another planet is compatible (and clearly life on earth is compatible enough for it to subsume it), why would it be "content" on whatever planet it originated from? We know nothing about its home on which to make that assumption. It's species may have had significant overpopulation problems, making an exodus from the planet very appealing. Prey on the original planet may have become sparse (very plausible considering how efficient and effective it is!).
and not be able to conceive of the idea of interstellar hunting.
This is directly contradicted by the movie. What is the first thing the Thing does when awakened? It starts hunting. Even without that evidence, why would we assume that it isn't smart enough? It's clearly extremely smart, and fully sentient. It can pass for human, including engaging in conversation (even if we assume Blair wasn't assimilated until after we see him saying he's "fine now", the video game, endorsed by Carpenter as a canonical sequel, does show infected/assimilated humans engaging in conversations yet still blending in.
How did it get a spaceship to come to Earth? A canon answer if possible.
As far as I can find, this was not directly addressed in any canon materials. However, there are two possibilities:
It stole the spaceship from another species, after assimilating the species. This would not be a stretch at all. Any species coming into contact with the Thing would likely be in extreme danger of assimilation.
It built the ship itself. Given that after taking over Blair, it was in the process of building a small new ship, this is not a stretch at all. If it can improvise an escape craft from scraps in anywhere from a few hours to a few days, why would we think it could not build a full ship given sufficient time and resources?
Problems with your question:
Based on John Carpenter's The Thing (1982), we have no way of knowing whether the spacecraft was built by the Thing, or by some other species of alien. All we know is that the Thing was on the spacecraft for some reason, and escaped when it crashed.
Even if the Thing is supposed to have built the spacecraft, we have no reason to think that it shouldn't have been able to do so.
There is no reason to believe that the Thing doesn't have an original form, but whether it does or not, this wouldn't affect its ability to build stuff.
The Thing can be a convincing dog, for god's sake. It must be more similar to an alien who can build a spaceship than it is to a dog, but it still knows how to whimper, and wag its tail, and lick people's hands to win their affection. The fact that it is a shape-shifter doesn't have anything to do with its ability to build stuff, any more than the fact that it is not a dog means it can't do dog stuff. If a Thing can go from being a dog to being a doctor in a matter of minutes or hours, why wouldn't it be able to build a spaceship, or a house, or a lawnmower? Its whole rason d'être is that it becomes a perfect copy of the people and animals it assimilates. That means that it can become a copy of a spaceship builder. If it could copy the appearance of a spaceship builder, but not the abilities of a spaceship builder, it wouldn't be a very effective copier, and it would immediately be recognized as a Thing.
To Answer the Question:
John Carpenter's The Thing (1982):
This movie itself doesn't tell us anything about where the Thing came from, who or what built the ship, or whether the Thing has an original form (as opposed to, for example, starting out as a blob of slime, then assimilating one creature after another).
Producer Stuart Cohen, in a fan site Q&A:
Q: During the opening sequence, the alien ship is seen to tilt suddenly as it approaches Earth. Was this intended to indicate something significant happening on board, such as a struggle over the controls?
A: No, only that the craft was having trouble...
The best online resource for information about The Thing is the fan site Outpost 31. Although it is a fan project, it is incredibly well made. In fact, when the prequel was being made, the crew needed to build the Norwegian base set and make it consistent with the original movie. They discovered that no one had preserved blueprints for the original set, and it was the fans at Outpost 31 who scrutinized the scenes from Carpenter's film and turned them into incredibly accurate blueprints, which the crew of the prequel used to design their set.
Here is what the Outpost 31 FAQ says about this issue:
Q: What was the Thing doing on the ship in the first place?
A: Well, there seem to be only four possibilities:
(1) The Thing was a member of the ship’s crew, perhaps even the pilot.
(2) The Thing was a passenger.
(3) The Thing was a stowaway.
(4) The Thing was part of the cargo.
The first idea would imply that the ship belonged to the Thing’s own race. Things were the ones who built and piloted the vehicle. For whatever reason, not many fans seem drawn to this notion. They tend to gravitate towards the more sinister possibilities. However, it is interesting to note that Susan Turner, the one who originally built the saucer model, makes an off-the-cuff remark on the DVD’s Terror Takes Shape documentary. There, she refers to the saucer’s central dome as the place “where presumably the creature is steering the ship.”
The second notion of the Thing being one of the ship’s passengers is even less popular. For whatever reason, no one has seriously defended this option. Perhaps it’s because people find it difficult to imagine the Thing as just a harmless passenger sitting in an interstellar cruise liner of sorts, comfortably sharing its cabin area with other space-going races.
The third idea of a Thing stowaway seems to be more attractive among the film’s fans. Perhaps the ship visited other places before coming to our Solar System, and on one of these planets it inadvertently picked up a Thing or two. Or maybe the Thing had gained access to the ship by assimilating one of its crew or passengers.
Surprisingly enough, the fourth possibility of having the Thing as part of the ship’s cargo or equipment is not without its supporters. Maybe the ship was on a scientific expedition to gather specimens from other star systems, and the Thing was collected from its own home world. Or even more: what if the Thing was part of the ship’s armament? The idea here is that the Thing was a biological weapon created by the race that built and operated the ship. Perhaps the craft was a military vessel equipped with the Thing as part of its standard weaponry. Maybe Earth itself had become a target for planetary invasion. But, as with all biological weapons, the creation ultimately turned against the creators.
The film obviously never tells us why the Thing was even aboard the ship, and so we’ll never know the answer for sure -- but it certainly is fun trying to come up with, as Fuchs would say, “one or two ideas.”
John Carpenter has been very taciturn about questions like this - he refuses to speculate on what the Thing was doing on the ship, what - if any - was the Thing's original form, who was a Thing, who wasn't, when people were "Thinged", etc. He seems to suggest that he isn't even interested in such matters.
However, we do see Blair-Thing building a new spaceship under his shack, and while it is nowhere near as complex as the original ship, it does prove that the Thing is capable of building such devices.
The Thing prequel (2011):
However, the later prequel, while possibly not canonical, does shed some light on the subject.
[Note: I have no real reason to say that the prequel isn't canonical, aside from the facts that Carpenter wasn't involved with it, and that it is mediocre at best, whereas the original movie is a masterpiece]
The original ending of the 2011 prequel reveals that the Thing didn't build the spacecraft. The ship was built and operated by a race of creatures called "the alien pilots". The alien pilot crew had been collecting specimens of lifeforms from various planets when they unknowingly brought the Thing onboard. It attacked them, and in an effort to prevent the Thing from spreading across the galaxy, the last pilot left alive intentionally crashed the ship in the most remote part of the nearest planet - Antarctica. The pilots all died, but the Thing survived, crawled from the ship, and froze.
In the original ending, Kate was to discover the original pilots of the spaceship which had all been killed by The Thing, which was an escaped specimen they had collected from another planet, implying that the ship was crashed in an attempt to kill the monster. "I liked that idea because it would be the Norwegian camp in space. Kate sees the pod room and one pod being broken, giving her the clues what happened. What didn't work was that she wanted to find Sander and stop the ship from taking off and still solve the mystery in the ship. These two energies were in conflict."
From the fan site Thule Station (quoting the director's facebook page):
In the pilot version Kate walks in this room and sees a dead pilot hanging. He was the last pilot alive and Kate sees that he killed himself because his air pipe was cut (basically Colin in space). The back story was that this Alien pilot race collected specimen from different planets and The Thing was one of them, broke free and killed all the Alien species in the ship (pod room).
I proposed to the studio a reshoot of a scene where Kate would wonder around in the ship and sees all the carnage caused by The Thing: an exterminated pilot alien race. The Thing, a specimen captured in a pod, broke free and killed the aliens on the ship. Or they killed each other not knowing who is who. The pilot in the pilot room kills himself and crashed the ship on Earth in the hope to stop The Thing.
In the version I intended to shoot, the ship was steered by an Alien race and The Thing was a captive that broke free and killed everybody on board. In the dead pilot version, the pilot controlled the ship with tubes stuck in his back, operating the ship through those tubes. The pilot kills himself and crashes the ship on purpose, hoping that it would kill the Thing. Of course it doesn’t, it climbs out and freezes himself.
So back to Kate. She sees the dead pilot and Sander, now has taken the form of the pilot (he has the genetics because of his spaceship slaughter fest 100.000 years before), has started up the ship. Sander attacks Kate in pilot form and corners Kate, who pulls her last grenade and threatens to blow them both up. That moment Carter runs in and sees what she is doing and blows up the sander Thing just to convince Kate that he is human. (he basically has no choice because he if fries Kate with his flamethrower everybody would blow up). Little complicated but we filmed this.
The Things (noncanonical fanfic short story by Peter Watts):
While not canon in any sense, this story makes it quite clear that the Thing retains all the memories and knowledge possessed by the life forms it has assimilated in the past. So, if the Thing had assimilated an alien who knew how to build spaceships, then the Thing would know how to build spaceships. If the Thing had assimilated Michelangelo, it would be able to produce beautiful works of art. If the Thing had assimilated me, it would be able to do everything I can do - for example, since I'm a chef, it would know how to cook very well.
And the Thing doesn't dump out all the information it obtained from the previous victim when it assimilates someone new - it just keeps adding everything it learns by assimilation. So if the Thing assimilated a spaceship-building alien, then Michelangelo, then me, it would be able to build spaceships, paint the Sistine Chapel, and make Chicken Cordon Bleu.
This is the way it seems to work in John Carpenter's The Thing as well. The Norris-Thing knew that Norris had a heart condition, and it feigned cardiac arrest to fool everyone into believing that it was really Norris. And more importantly, as mentioned above, the Blair-Thing knew how to build a small flying saucer - not because Blair knew how to do it, of course, but because one of the Thing's previous victims knew how to do it.
Because The Thing used to be a lot more advanced before the crash.
Of course, this is speculation as none of the primary source materials - the 1982 movie itself or Campbell's original novella - go into any real detail about The Thing's origin. However, Peter Watts wrote an award winning short story called The Things from the alien's point of view. Early in the story, it is clear that the creature we see in the novella and movies is a heavily damaged version of its previous self:
I was so much more, before the crash. I was an explorer, an ambassador, a missionary. I spread across the cosmos, met countless worlds, took communion: the fit reshaped the unfit and the whole universe bootstrapped upwards in joyful, infinitesimal increments. I was a soldier, at war with entropy itself. I was the very hand by which Creation perfects itself.
So much wisdom I had. So much experience. Now I cannot remember all the things I knew. I can only remember that I once knew them.
I remember the crash, though. It killed most of this offshoot outright, but a little crawled from the wreckage: a few trillion cells, a soul too weak to keep them in check. Mutinous biomass sloughed off despite my most desperate attempts to hold myself together: panic-stricken little clots of meat, instinctively growing whatever limbs they could remember and fleeing across the burning ice. By the time I'd regained control of what was left the fires had died and the cold was closing back in. I barely managed to grow enough antifreeze to keep my cells from bursting before the ice took me.