The Princes of the Plains are minor antagonists in the Hitchhiker's Guide novel Life, the Universe and Everything.
They appear at first claiming they are "noble" Princes who live high in the Plains and they seek to kill the Forest Dwellers, who they see as infidels. The Princes have a pantheon and when they see the Heart of Gold spaceship they believe it to be a divine intervention telling them to set about the Forest Dwellers, and the Tribesmen of the Cold Hillsides, who they hate. The Princes butcher the Tribesmen, then race through the Forest, killing every Dweller they meet.
The Forest Dwellers send messengers to the Princes, asking to stop the madness but they have poor memories. The reason behind the butchery seems very good and reasonable, but the Forest Dwellers forget the Reason when they get back home, and it is a great relief when the Princes butcher each of them.
It is unknown what happens to them after this although they were apparently sued along with the Tribesmen however this is seen as a poor punishment