It's the stallion you get in a mission with Bonnie. She takes you out to brake some horses, and it's the golden one you brake at the end of the mission. After this, it appears as a deed purchasable in shops for $750.
The reason to own the deed is that if the one you broke dies or is otherwise 'lost' (like accidentally 'saving' another horse by hitching it), you can use the deed to call it again. The horse deeds are infinitely usable. It's not a one-use deal.