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]