This mod adds the ferret pet to the game, both as a permanent transformation fish and as a pet selection when making a new character.
Runic made the pet and included it in the TorchED files. All I did was copy the files and make a fish to turn your pet into the ferret.

While the Catfish and Dogfish are only sold from the general items vendor from level 15 onwards, the Ferretfish is available from level 1.

If you have the Merchant Pack mod installed, the Ferretfish will be sold by Arthur the Fishmonger, near the town's fishing hole.