realMyst has a collection of critters that do not appear in the original version. 3d animals appear in Myst Island, Selenitic, Channelwood, Stoneship (underwater), and Rime (from afar). I have not seen any animals in the Mechanical Age. Below, I present some images of these animals, including some pictures contributed by Gehn and 75th Trombone.
I've also included some "cross-dressing" images. These are based on a method described by lonelyto25, who described a way of replacing the "shape" of one animal with another's.
Myst Island Seagull Butterfly Butterfly Cross
Selenitic Bat Bat Cross
Channelwood Frog Stoneship Fish Sole Globefish
Note: In the middle picture, there is a "V for Victory" plant -- you'll see what I mean.
Jellyfish Ray Eel Tortoise Fish Cross Dressers Rime Whale Thingy Small Shark