A critter is an animal. If you hear scratching noises in your ceiling at night, you can be sure that some kind of critter is living in your attic.
Critter is an informal word for any living creature, from a wolf to a spider. You might like going to watch various critters at the zoo, or live on a small farm where you can keep critters of your own, like goats and chickens. Critter first appeared in the early 19th century, a shortened slang or joking form of the word creature.