The Tsemu was an animal native to Nimbus III. It resembled a stockier and shaggier Terran Horse with a horn on its snout. The Tsemus was used as a riding animal and as beast of burden by the planets inhabitants. The settlers from the Federation started calling the Tsemus "horses" and the name stuck.
Tsemu were known to be docile and benevolent beings with limited intelligence. Homesteaders who had turned into raiders on Nimbus III would also slaughter Tsemus as food. Tsemus were susceptible to the Vulcan nerve pinch.
In 2287 Sybok and the members of the Galactic Army of Light used Tsemus in their assault on Paradise City. Later when Captain James T. Kirk led an assault team to rescue the hostages held by the Galactic Army of Light, they confiscated some Tsemus. (TOS movie, novelization & comic adaptation: Star Trek V: The Final Frontier)