The Spirit Airlines pet policy is as follows:
Small domesticated pets are accepted in the aircraft cabin on all domestic flights including Puerto Rico and St. Thomas, U.S.V.I.
Spirit Airlines does not accept animals on international flights, except in the case of service animals.
Spirit will only allow the following pets on the aircraft: domestic dogs; domestic cats; small household birds (except to/from Puerto Rico and the U.S.V.I. ); and small domestic rabbits (except to/from Puerto Rico and U.S.V.I).
Birds are not allowed to and from cities in Puerto Rico or the U.S.V.I.
Birds that are not considered household birds include farm poultry, waterfowl, game bird, birds of prey, and flightless birds.
Exotic pets such as snakes, spiders, rodents, etc. will not be accepted.
Spirit Airlines does not transport pets in Cargo at any time.
At the airport find pet relief areas at U.S. airports.
More information on the Spirit Airlines pet policy