Professor Firenze is a centaur who is one of two Divination teachers at Hogwarts.
Firenze first appeared in Harry Potter and the Philosopher's Stone, when he saved Harry Potter, who was serving a detention in the Forbidden Forest, from Lord Voldemort. He is absent from the next three books, but in Order of the Phoenix, Albus Dumbledore hired him as a Divination teacher when Dolores Umbridge sacked Sybill Trelawney. This resulted in Firenze being cast out of his herd, since agreeing to work for a human is considered degrading for a centaur.
Firenze continued to teach Divination alongside Trelawney in Half-Blood Prince, causing Trelawney much consternation, as she considered it an insult to be sharing classes with a "horse".