Horses

Horses are large fleet-footed mammals who are frequently employed as domesticated beasts of labor by humankind and as food sources by gargoyles. Native to a number of worlds, including Britannia, Serpent Isle and Earth, these generally docile creatures appear in Ultima I,  II,  III,  IV,  V,  VI,  VII and  VII Part Two.

Description
Riding horses were commonplace in the earliest days of Sosaria, and were frequently hired by travelers seeking to quicken their journeys across the often perilous wilds of the realm. While horse husbandry appeared to fall into a slight decline during the era of Exodus, canny adventurers could still locate and subdue riding steeds should they travel to the distant lands of Ambrosia, where herds of wild horses still roamed the grasslands (although these swift Ambrosian mounts would frequently require either powerful magic or hard-to-locate items to tame). Eventually, however, with the end of the Age of Darkness, groomsmanship slowly reemerged as a profession, and horses were a common component of day to day Britannian life for much of its history - serving to plough fields, draw carriages and convey travelers for many centuries

The Horse Plague
Sometime during the catastrophic years of the Imbalance and Great Cataclysm, a far reaching and deadly plague affected all the equine species of Britannia, killing all horses throughout the realm. By the time of the Avatar's last journey through the realm, such poor creatures could often be found hanging outside of the boundaries of towns and cities. While a woman known as Cleo was arrested and imprisoned for the creation of this contagion, its exact origin and vectors of travel were never made clear.

Notable examples

 * Smith the Horse: a talking stallion often found living on the grounds of Iolo's Hut near Yew; famous for giving slightly out of sync advice to the Avatar

Trivia

 * In the Ambrosia of Ultima III, horses could be "tamed" by stopping time, using either a powder or the Altair spell, and then mounted. Due to an in-game bug, this had the curious side-effect of duplicating any horse captured in this fashion (though the duplicate horse appeared as a grass tile while still frozen).
 * In Ultima V various stablekeepers such as Kurt, Kraw or Treanna can describes differences between the various types of Britannian horses, identifying plough horses, mountain horses, steppes horses, and Valorian warhorses as distinct breeds.
 * When the C64 port of Ultima VI was created, the limits of the system proved difficult to implement horse riding. As a result, horses were removed from the game, with the in game explanation that the raiding gargoyles had eaten them all (with exception of Smith). Similar technical difficulties also resulted in the removal of horses in the NES port of Ultima V, although in this game the explanation used was that the horse population had been decimated by hungry people.
 * In the SNES Port of Ultima VII, the Guardian will taunt the Avatar should they kill a horse, encouraging them to "Stop horsing around!"