Appa's mother

Appa's mother was the flying bison who gave birth to Appa and four other flying bison. She resided in the Eastern Air Temple. She was very large compared to Appa and the other baby sky bison, who were less than one tenth of her size. Even so, when Aang and Appa first met, Aang was already slightly shorter than the baby sky bison who would become his future companion.

History
Appa's mother appeared to Appa in a dream when the latter was feeling alone. She was flying along at the Eastern Air Temple, with her five calves circling around her, heading toward Sister Iio and several Air Nomad children, among whom Aang. She nuzzled her children tenderly, before willingly directing them to the platform where the Air Nomad boys were awaiting them.

Trivia

 * Appa's mother is missing the tip of her right horn.