I have 2 ideas about what is going on with animals; either they are bugged or we are witnessing evolution!
If you take the “bugged” approach, animals appear to be harder to find at the moment because they will just wander off into the water, walking around the sea bed (I would to if a bunch of naked men were trying to kill me with a rock and eat me). Occasionally when you hit them with a rock they will give would instead of dying, as it trees. Also when dying it has been my experience that every animal type reverts to the wolf death animation and loot.
But hold on just a second. Are trees not the oldest form of life in the Rust universe? Ever since wolves first appeared their has been the chance of them providing wood instead their normal functions. In fact the earliest wolves were stationary, much like trees…could this be evidence of a transitional form? Wolves were the animal, and every animal since shares behaviors, and ultimately death animations, further evidence of evolutionary speciation events? Currently the animals and simple and stupid, and only superficially different from one another. However if my evolutionary theory hold I predict we shall see a rise in intelligence as speciation continues. Members of individual species will evolve unique death animations and behaviors, some aggressive some not. Most will stop walking under water on account of lungs. But it just might be possible for fish to evolve from our land animals given the right circumstance and a little bit of luck.