Why did they all look like this?
OK, OK, OK, I realize that the main plot to the story involved Arnold schwarzenegger as the actor so it had to look like this.
But my query is that in the first Terminator, they used dogs to detect terminators and the one that broke through the defenses looked nothing like the other T-800's,
So was there anything canon that stated why all the Terminators that were sent back looked the same?
The question of why a T-800 Model-101 (Arnie-type) Terminator was chosen is different in each of the films.
The T-800 in this film is a top-of-the-line model. It's literally the acme of what Skynet can produce and while there are indeed other models seen (particularly in the flashback sequence), the Arnie version is likely the most lifelike out of the bunch.
REESE: Pay attention. The 600 series had rubber skin. We spotted them easy. But these are new. They look human. Sweat, bad breath, everything. Very hard to spot. I had to wait 'til he moved on you before I could zero him.
It's circular, but John Connor chose to send back an identical model to the one that attacked his mother because he (as an adult) remembered being saved by that model as a child
Connor specifically looked for that model to send because he remembered him as such and wanted to make sure that he's doing what he was suppose to do - send the T-800 that he remembers back in time to 1995 so he can protect him from the T-1000. He knew that for things to go the way they are suppose to, he has to find the terminator that looks like the one that he remembers.
There's no good indication of why Catherine Brewster chose to send back that particular Terminator other than that it may have been the only upgraded T-850 (one with additional infiltration software) that they had immediate access to.
We do know that the 'Arnie' exterior was chosen by Skynet because of John's "emotional attachment" to that particular model. It's not a stretch to imagine that Catherine probably made the same judgement when she was deciding to send him back.
"John Connor was terminated on July fourth, 2032," Terminator said. "I was selected for the emotional attachment he felt to my model number, due to his boyhood experiences. This aided in my infiltration."
Terminator: Rise of the Machines - Official Novelisation
Two T-800s are seen in this film, one in the opening sequence (presumably chosen by Skynet for precisely the same reason as in Terminator) and one that was sent back by persons unknown to protect young Sarah Connor. We have no good intel on why this model was chosen (or who it was sent by) but I think we can assume that the fact that it's proven itself to be a reliable protector is a key factor.
Terminator 3 had a deleted scene that explained Arnold as basis for T-800s.
EDIT: My answer is based off the TC first giving out of universe scenario:
OK, OK, OK, I realize that the main plot to the story involved Arnold schwarzenegger as the actor so it had to look like this.
and then based specifically on the actual question, and thought they were more or less wanting an in-universe answer.
So was there anything canon that stated why all the Terminators that were sent back looked the same?
The future 800 series model terminator in the dream sequence was never sent back.
http://terminator.wikia.com/wiki/Future_Terminator
If question was asking more why OTHER 800 series models weren't sent back instead of Arnold, the answer varies from movie to movie and has also been answered based on that framing of the question.
T2 Illustrated Screenplay
Extended Future War Sequence - 5/10/90 draft
INT COLD STORAGE ROOM
Connor walks into the darkness, followed by a few technicians. They are in a vault-like cold-storage room. Hanging in steel racks from ceiling tracks are hundreds of what appear to be men. They are in rows of ten. Within each row, each of the bodies are absolutely identical.
Connor signals the techs to remain by the door and walks out among the dark bodies. They are UNACTIVATED TERMINATORS. He stops at a row in which they are identical to the terminator which was sent to kill Sarah (the Arnold model).
He walks to the end of a row. There is one empty rack. He faces the terminator in the next rack. Its eyes are closed.
John seems distant as he studies that face.
(source)
There's only two Terminator movies and a re-boot. That's why 'every' terminator sent back is a 101 model, other than the fact that as Kyle said, "these are new."
