Appa sky bison

Appa first met Aang at the Eastern Air Temple while still a young calf. Young airbenders, usually six years old, were allowed to meet their future companions, who would bond with them for life. Not much was explained about the exact nature of the relationship between the Air Nomads and their flying bison, but it appeared that the herd's mother willingly directed her calves to meet the young monks.

Aang came forward and offered an apple to Appa, and almost immediately afterward, the two became best friends and were together for most of their lives up until Aang's death.[1]