(Seal of Darius the Great, showing a Caspian horse)
I should add that it's interesting this horse was apparently buried alongside humans. This is not uncommon for ancient burials and demonstrates just how much horses were valued, both then and now.
It's also no wonder the royals love Caspian horses. They are smaller than modern horses, about 2/3 the size, but are known, as the famous late horse breeder Louise Firouz once said, as being "kind, intelligent and willing." CAIS additionally describes Caspian horses as having "light frames, thin bones, short, fine head with a pronounced forehead, large eyes, short ears and small muzzles. They are very fast, and incredibly strong and spirited, but also have good temperaments."
The Caspian sounds like an appropriate horse for newlyweds Prince William and Kate Middleton to ride off together into the sunset on.