D-Horse is a supporting character from "Metal Gear Solid V: The Phantom Pain".
During the attack at the hospital shortly after Venom Snake had just woken up from his nine year coma in 1984, Ocelot brought D-Horse over and rescued Snake, finding him crawling out of an overturned ambulance. Snake then rode the horse with Ocelot as he fended off a fiery winged unicorn-like specter piloted by a ghost from his past. Eventually, the horse and its occupants stumbled upon a collapsing cliff from lightning, although the horse survived.
Sometime later, Snake rode D-Horse over to Afghanistan, accompanied by Ocelot on another, brown horse, in order to rescue Kazuhira Miller, who had been ambushed by Soviet forces. Snake then went ahead with D-Horse as his only partner. After rescuing Miller, D-Horse was extracted via a Fulton recovery balloon and could be deployed as a Buddy at any time.