There are several different questions you need to ask yourself when creating a creature besides what the animal looks like. A strong basis in biology is helpful, but not needed. (I find my MS in Biology very useful.)
These are the questions I ask myself when I first start out making a new creature.
1) What kind of creature is it? (Cat? Dog? Fish? Bird? Mythical? Elemental? Etc.)
2) Where does it live? (Trees? Cliffs? Ocean? Stream? Lake? Pond? Forest? Mountains? Desert? Etc.)
3) What can it do? (Jump really high? Fly? Swim? Climb? Telepath? Magic? Etc.)
5) Can it be tamed? (Ridden? Kept as a pet? Pull a cart? Etc.)
6) Is it dangerous? (Poisonous? Predator? Really big for trampling? Etc.)
7) What does it look like? (Furry? Scales? Rough skin? Feathers? Etc.)
These seven questions can be considered the bare bones of your creature. As you build and describe it more you'll find more questions to ask and more things to describe.