From Halopedia, the Halo wiki
The spadehorn is a bovid indigenous to the planet Reach. A hardy herbivore, the animal named for its pair of flat-tipped spade-like horns. When humans settled Reach, the spadehorn was domesticated.
Behind the scenes
The Spadehorn was originally to appear in Halo: Reach as ambient wildlife. Concept sketches were created for three different varieties of the creature; the Highlands Spadehorn, Prairie Spadehorn and Angelhorn, each with different shaped horns. In Halo Mash-Up: Minecraft Evolved, sheep mobs sport horns reminiscent of the Highlands Spadehorn.