The three-horns deer (or just three-horns) is a fictional species of cervine based on Synthetoceras. They're one of the two species of animals that are commonly herded by the Clan of the Named, together the dapplebacks horses.
It's said that the wild three-horns were first tamed and introduced in the Clan herding system by the former Clan leader Baire.
The three-horns deer is based of the extinct animal called Synthetoceras, which was the largest of its family. It was also the last of its family, and it has a incredible y-shaped horn. Only the male had this horn.
Ratha: "Why must we have three-horns in the herd? They're hard to manage. They fight among themselves and bully the other animals."
Thakur: "They are larger and yield more meat. They have more young. And they are harder for the raiders to kill and drag away."
~Ratha's Creature page 6.