The horse (Equus ferus caballus) is one of two extant subspecies of Equus ferus. It is an odd-toed ungulate mammal belonging to the family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, Eohippus, into the large, single-toed animal of today. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses serve in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as warfare, police work, agriculture, entertainment, and therapy. Some cultures eat horses, though in others this is taboo, whether officially or unofficially.
Horses are present in nearly every Harry Turtledove work, usually in their millennia-old OTL role of pack-animal, livestock, or warhorse. Only in a few stories is the species' presence (or absence) relevant to understanding how a particular world works.
Horses did not exist in the world where Georgia and Lani lived. Instead, people rode on rabbits which were the size of horses, not that a person from that world could make any sense of this comparison.