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 Dressers
Selenitic	Bat
	Bat Cross Dressers
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