Meadow vole

Meadow Voles are mice-like creatures that live mostly in forests (their names are misnomers). Being rodents, they are likely to inhabit many other areas as well. The vocal sounds of meadow voles are small squeaky sounds, and they are friendly towards humans.

A meadow vole was first seen in Winter Solstice Part 1: The Spirit World when Iroh is relaxing in the forest.

One was also seen in The Blue Spirit when Momo carried one back to a sick Katara and Sokka instead of water.

Winter Solstice Part 1: The Spirit World.