Appa has been Aang's sidekick since Aang was six years old.
Aang was six years old when he met Appa. We see him as a young bison when he first meets Aang. Aang offers him an apple, and they become friends for life.
Appa was with Aang when Aang ran away from the Air Temple. Monk Gyatso was ordered to be separated from Aang, so Aang decided to run away. Him and Appa were flying, and got stuck in a storm. As a wave sweeps Appa and Aang from the sky, a strong undercurrent pulls them deep into the ocean. Before perishing, Aang enters the Avatar state, and encases himself and Appa in an iceberg.
Appa’s Personality
Appa adds a lot of subtle humor to the show. He can’t talk, so his humor is channeled through body language and grunts. He is always hungry. Whenever we see him getting fed, he wolfs down gargantuan quantities of food in one bite. Appa appears comfortable and even-kealed all the time. Even in battle, he seems to just grunt and keep plugging away, with the same relaxed look on his face. He also likes to sleep a lot. While Appa jumps into action at the sound of Yip Yip, he appears to be generally lazy. Any time that the group takes a break, Appa rolls over or buries his face in his paws, and goes to sleep.
Appa gets tired often, and needs to take breaks!
Appa is a flying Bison, so he obviously has the ability to fly. Appa apparently has the ability to airbend in a manner which allows him to fly. While he is up in the air, he controls himself with his massive flying tail. He also has six legs. It requires a lot of effort for him to fly, so he must take breaks every so often. The group often jumps from place to place between episodes. In addition to being comfortable flying around in the air, Appa is a fairly good swimmer. In The Avatar Returns, it takes a little bit of time for Katara and Sokka to discover “Yip Yip”. They are forced to ride Appa in the water until Sokka remembers Aang saying “Yip Yip” to get Appa to fly. Appa waves his tail up and down in the water to move himself along.