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Thingy would probably fit as a Killer Rabbit, considering it didn't attack until it was threatened. There's nothing saying that a Killer Rabbit is inherently evil.
RESET:
How about An Aesop that is presented in said episode? It's good ol' "Don't judge book by it's cover", but sort of Deconstructed and Reconstructed: Most shows present us a pet that turns out to be a monster and teach not to say that cute things must be harmless. "Thingy" episode, on the other hand shows a pet that turns into a monster and we are taught that ugly monsters can still be kind hearted.
Can I say it's Reconstructed while Deconstruction sort of never happened?
Because it felt so different for me. It is a really good piece of work, but remains forgotten. I want to describe it in most appealing way possible (sticking to the truth, of course), so that anybody who reads this describtion understands that this show deals with matters similar to other animations, but resolves them in it's own way.
You can still say that. That's the point of having examples. The trope can be played straight and still be unlike most other works utilizing the trope.
Link to TRS threads in project mode here.

In one episode of Super Robot Monkey Team Hyperforce Go!! there is a cute critter that turns out to be a dangerous, shape-shifting monster. However, eventualy it proves that dangerous doesn't necessairly mean "evil" and saves the day.
I am pretty sure that's some play with Killer Rabbit trope, but I'm not sure which one:
Is it an inversion, because it's a giant not-cute animal that turns out to be peacful? Is it a subversion, because critter is hinted to be a killer (by his appearence) and then revealed to be harmless after all?