Zaetar was a keeper of the grove, the eldest son of Cenarius. He was seduced by the demon-goddess of the earth Theradras, and taken to Maraudon. The product of their union was the centaur, a twisted mockery of the beauty of the dryads and keepers of the grove. According to the story, the first act of the jealous and evil centaur was to kill their father. Theradras was so dismayed by this that she kept Zaetar's remains, and his soul, so that they could be forever locked away with her in Maraudon.
You may encounter Zaetar's Spirit in World of Warcraft.