The Collaborative International Dictionary of English v.0.48:

critter \crit"ter\ (kr[i^]t"t[~e]r), n. [from creature.] [Also
   spelled crittur.]
   1. Any animal; as, lots of critters come out only at night.
      [U. S., western dialect]
      [PJC]

   2. Specifically: A domestic animal or a non-predatory wild
      animal; -- contrasted with varmint, also dialectal. [U.
      S., western dialect]
      [PJC]

WordNet (r) 3.0 (2006):

critter
    n 1: a regional term for `creature' (especially for domestic
         animals)

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "critter":
   Brahman, Houyhnhnm, Indian buffalo, an existence, animal, aurochs,
   beast, beef, beef cattle, beeves, being, bison, body, bossy,
   bovine, bovine animal, brood mare, brute, buffalo, bull, bullock,
   calf, carabao, cattle, charger, colt, courser, cow, creature,
   creeping thing, dairy cattle, dairy cow, dobbin, dogie,
   dumb animal, dumb friend, entelechy, entire, entire horse, entity,
   equine, filly, foal, gelding, heifer, hornless cow, horse,
   individual, kine, leppy, life, living being, living thing, mare,
   maverick, milch cow, milcher, milk cow, milker, monad, mount,
   muley cow, muley head, musk-ox, nag, neat, object, organism, ox,
   oxen, person, persona, personality, prancer, something, soul,
   stallion, steed, steer, stirk, stot, stud, studhorse, tarpan,
   thing, top horse, unit, varmint, war-horse, wild horse, wisent,
   yak, yearling, zebu