"Aang, Momo-3, and the unnamed polar bear—dog. Unfortunately, the latter never made it into the show, but we always really liked him and have a feeling he’ll turn up somewhere…"
OK, that’s a caption of very early ATLA concept art. Before Bryan Konietzko came up with flying bison, he though of a polar bear-dog.
GUYS, NAGA IS THE POLAR BEAR-DOG.