The Ferret is an exclusive pet included with the retail box and GOG edition of Torchlight (however, it can also be implemented with mods). It functions like every other pet in the game. The player can choose the Ferret (in addition to the Dog or Cat) at the title screen when creating a new character.
Unlike the Dogfish and Catfish, there is no fish to transform it back into a ferret, should you transform it into something else permanently (although this also can be implemented with mods). The Ferret is the only pet in the first game to be able to stand on its hind legs.