From Halopedia, the Halo wiki
A thorn beast is a type of docile herd creature that is used for food and other resources by the Covenant and the Jiralhanae in particular. In the wake of its collapse, Jiralhanae have been known to still make use of the them.
Thorn beasts are named for the five "thorns" that ridge their spines. A thorn beast's head and neck contain high concentrations of neurotoxins; likely a defensive adaptation against predators. Thorn beasts have often been associated with Jiralhanae. Chieftain Maccabeus enjoyed eating the ball of muscle under the smallest of the beasts' thorns when it was properly prepared, and the commander known as Thrallslayer enjoyed it immensely, saying that it was "both an appetizer and a dessert". Thorn beast is served with the spines pointing upwards and legs splayed outward. The head and neck are removed prior to consumption in order to avoid ingesting their neurotoxins. Entrails are often used to make dipping sauces. When the Jiralhanae vessel Valorous Salvation landed on Beta Gabriel and the Jiralhanae began hunting the humans there, it was remarked that one Brute named Brother Golubus came close to making the fatter humans taste like thorn beast. In addition to being eaten, thorn beasts are also a source for leather. The Chieftain Hekabe used cloth and thorn beast leather to wrap a Forerunner artifact in his possession.
Thorn beasts were originally meant to appear in Halo: Combat Evolved, but were one of many creatures cut from the final game. The thorn beasts were going to be rideable creatures for Marines before they were cut for AI issues.
They were later intended to appear in Halo Wars, appearing in the E3 2008 build of the game. The developers stated that they would have reacted to units and battles around them. In the videos however, they merely ran away from units, not battles; if there was a bombing next to it, it wouldn't have flinched. However, since the videos were made as the game was still in development, this lack of a reaction may have been changed for a more real gaming experience if the creature would have made the final cut. The creature was ultimately one of many such creatures cut from the development of Halo Wars.
In Halo Mash-Up: Minecraft Evolved, some horse mobs are retextured to look like the Thorn Beast.
List of appearances