Did you immediately picture a cold-blooded horse sunning itself on a rock like a snake? When horse people use these terms, they are not talking about the animal’s ability to warm itself. Instead, it speaks to temperament traits.
A cold-blooded horse is usually a draft-type breed (think Clydesdale, Gypsy, Shire, etc.) that is known for a heavy build and a calm temperament. Conversely, a hot-blooded horse is more refined (think Arabian, Thoroughbred, etc.) and are high-energy, high-spirited horses.
A warm-blooded horse is a horse that is the result of breeding a cold-blooded breed with a hot-blooded breed. They even have separate registries for certain popular types including the Hanoverian, Dutch Warmblood, American Warmblood and the Irish Draught.