Black stallions

From Glen Cook Wiki

AKA Horses of Charm, a special breed of magically enhanced horses developed personally by Lady in the fullness of her powers at Charm. The horses are enormous, glossy black, and possessed of incredible stamina and speed; their eyes glow red, and their hooves strike sparks when they run. A Horse of Charm can run at a flat gallop for days faster than any normal horse and not tire out. They are also remarkably intelligent. Their lifespan is around forty years, according to Lady.

In the Northern Empire, these horses are considered extremely valuable and only given to very favored leaders. When Lady abdicates and heads south with the Black Company, she brings a team of six such horses along with one of her iron carriages. They succumb to injury over time, until the last two are stolen by Soulcatcher after she traps the leadership in the Glittering Plain. By the time of Water Sleeps, only one remains; it is with Mogaba and then brought back to Taglios so Soulcatcher can have ready access to it. This particular horse, however, is the one that Sleepy used to ride. It recognizes Sleepy (in disguise!) when Mogaba's cavalry passes her on the road to the Shadowgate. The horse actually breaks out of its stall in Taglios and runs hundreds of miles to rejoin Sleepy.