Appa is Avatar Aang's huge sky bison, a furry white flying creature with six legs and the ability the carry several human passengers.  Appa cannot speak English, but can express his emotions through growls, grunts, and low purrs.

At three points in The Promise, the script specifically calls for Appa to say, "Raaar!"  However, there are many scenes which could benefit from the ambiance of Appa sounds.  In the TV series, Appa was played by Dee Bradley Baker, who has a long career creating unique animal voices -- as well as having played an extraordinary number of human characters with actual words to speak!  Here is a sample of his work: https://www.youtube.com/watch?v=FylESKJMGx0