What animal is Bolin's pet?

Bolin's pet is a fire ferret named Pabu. Pabu is a small, energetic animal who becomes a recurring companion to Bolin and his older brother, Mako, throughout The Legend of Korra.

Bolin and Mako encounter Pabu while living as street youths in Republic City. The story of how they rescue him from a pet store is explored in “Republic City Hustle,” a set of animated shorts released before the second season. Pabu quickly becomes part of their extended group and often provides comic relief during dangerous adventures.

Although Pabu is called a fire ferret, his design is a fictional hybrid inspired by a red panda and a black-footed ferret. He is not a badgermole, sky bison, or polar bear dog. Those animals belong to other parts of the Avatar world: badgermoles are associated with earthbending, sky bison with the Air Nomads, and polar bear dogs with the Water Tribes.

Pabu's name and species are sometimes overshadowed by his expressive behavior, but “fire ferret” is the canonical answer to the question.