Sorry but wtaf was that

He was a dog

Well from early on they explained the mutants as regular people that started developing the animal traits so i think that applies to fluffy too (like k and m)

We don't know for sure, but in past chapter you can see a black "normal" dog in the farm could be him, and I think he is more "aninal" than the humanas who develop animal traits, like I think he can't eve speak much