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.
The Spadehorn was originally to appear in Halo: Reach as ambient wildlife.  Concept sketches were created for eight different varieties of the creature; the Highlands Spadehorn, Prairie Spadehorn, Angelhorn, Ringhorn, Hammerhorn, Pryhorn, Devilhorn, and Godhorn, each with different shaped horns. Spadehorns were intended to have an unusual genetic adaptation - horns which are flat and tough rather than brittle and pointy like those of their Earthbound cousins. The shovel-like horns were intended to dig through the rocky or frozen soil in Reach's boreal regions, though some variations were used instead to carve moss or lichen from sheer rock, or strip the bark from Reach's indigenous trees.  In Halo Mash-Up: Minecraft Evolved, sheep mobs sport horns reminiscent of the Highlands Spadehorn.