A colloquialism referring to any animal. It's primarily used in America and Ireland. It's what's known as a pronunciation spelling of "creature" -- in fact, it probably dates back to the 16th and 17th centuries or before, when the "-ture" suffix was still pronounced without the "ch" sound.

Some critters include the cougar, the coyote, the grouse, the horse, the water buffalo, the mosquito, the sheepdog, the rattlesnake, the bobcat, the deer, the Siamese cat, the longhorn, the canary, the rat terrier, the roadrunner, the hoop snake, the field mouse, the chicken, the alligator, the sheep, the fox, the brown recluse, the kestrel, the moose, the rabbit, the beagle, the rabbit, the swan, the pig, the zebu, the pit bull, the owl, the gopher, the preying mantis, the donkey, the caribou, the plover, the vampire bat, the duck, the scorpion, the bison, the hound dog, the trout, the ostrich, the earthworm, the giraffe, the cobra, the finch, the elk, the ox, the jellyfish, the poodle, the camel spider, the heifer, the snipe, the crocodile, the penguin, the dachshund, the honeybee, the yak, the jackalope, the mandrill, the killer whale, the sparrow, and the werewolf.

Some research dug out of www.dictionary.com