How did the Basilisk decay so rapidly in the Harry Potter film?
Only 5 years passed from CoS to TDH Part 2. It should have had flesh left on it, but all they show are only bones. The chamber is not said to have bugs, creatures or anyone alive within it, so the flesh can't have been eaten.
When Ron and Hermione bring Harry the basilisk fangs in the book, the dead body of the basilisk isn’t seen or described in detail. It’s unclear in the book what state of decay the basilisk was in, it’s just said that Ron and Hermione took the fangs from it.
“Harry’s eyes dropped to the objects clutched in Ron and Hermione’s arms: great, curved fangs torn, he now realised, from the skull of a dead Basilisk.”
- Harry Potter and the Deathly Hallows, Chapter 31 (The Battle of Hogwarts)
They don’t describe the dead basilisk to Harry, and the reader isn’t “there” when they remove the fangs. In the book, there’s nothing indicating if the dead basilisk was a skeleton or a rotting corpse.
Basilisks may not be equivalent to “normal” creatures - it’s nowhere near certain that any individual type of magical creature, unless it’s specifically mentioned, would decompose in the same way as “mundane” creatures. Magical creatures do many things mundane creatures cannot - as just one example, the Ashwinder, another type of magical serpent, is created by fire rather than being born in any typical manner, and collapses into dust when it dies.
The Ashwinder lives for only an hour and during that time seeks a dark and secluded spot in which to lay its eggs, after which it will collapse into dust.”
- Fantastic Beasts and Where to Find Them
We can’t reasonably presume that a basilisk would decay in the same way as any mundane creature. We also don’t know if the same sort of creatures that would consume mundane animal bodies would be even able to consume the body of a creature like the basilisk - this may be different as well.
To show the dead basilisk on-screen, the filmmakers may have considered it better to show a skeleton rather than a decaying body. This could be for many reasons, for example to create visual impact, or possibly because they considered showing a decaying body to be needlessly gross.
In the books there were the remains of small animals within the tunnel leading to the Chamber itself:
But the tunnel was quiet as the grave, and the first unexpected sound they heard was a loud crunch as Ron stepped on what turned out to be a rat's skull. Harry lowered his wand to look at the floor and saw that it was littered with small animal bones.
Harry Potter and the Chamber of Secrets, Chapter Sixteen - The Chamber of Secrets
While this isn't direct evidence of living creatures down there, it seems unlikely that there were none at all. The basilisk didn't appear to have free access to the tunnel leading up to the Chamber, which raises the question of where all of these animal bones came from; the most logical conclusion seems to be that they're the primarily corpses of living animals that happened to die - from whatever cause - within the tunnel.
Small animals can often gain access to places that humans - or a huge basilisk - can't. Hogwarts - and the Chamber of Secrets - were at least a thousand years old, so there were likely gaps sufficient to allow small animals to get in and out of the Chamber itself, and to feed on the corpse of the basilisk.
The light slid over a gigantic snake skin, of a vivid, poisonous green, lying curled and empty across the tunnel floor. The creature that had shed it must have been twenty feet long at least.
Only 5 years passed - the basilisk should have had flesh left on it, but all they show are only bones.
Decomposition of dead bodies happens a lot faster than that, even for large animals. It takes about a year until skeletonisation, whether for a pig or for an elephant. I don't think it would be much different for a large snake - and even for a magical snake we should assume that any magical self-protection fails upon its death.
The chamber is not said to have bugs, creatures or anyone alive within it, so the flesh can't have been eaten.
It's not said to have no bugs or living creatures either. It's a large cave, unlikely not to be accessible from the outside. In the books, rats are mentioned explicitly - and even if the basilisk preyed upon them, it doesn't any more after its death.
Even assuming there are no bugs (or larger animals), the microfauna of the basilisk itself is well capable of destroying it from the inside. In the films we can also see large amounts of water in the cave, which might have slowed down decomposition, but it was at least partially exposed to the air. Ron and Hermione get the fangs from a dry skeleton at least.
It was eaten, just not by other lifeforms, this assumes that Basilisks use a Cytotoxic venom that they have limited immunity to, said immunity being based on a stomach lining like mucus membrane that must be continuously replaced and which separates the toxin from the rest of the Basilisk. As such once the creature is dead its own venom rapidly leaks out of its venom sacs and spreads through its soft tissue and dissolves it from the inside out. It could decay to a skeletal state in just days.
