D-Horse

Diamond Horse (also known as D. Horse) was a white horse owned by the PMC Diamond Dogs during the 1980s.

During the attack at the hospital shortly after Big Boss had just woken up from his nine year coma in 1984, Ocelot brought Diamond Horse over and rescued Big Boss, finding him crawling out of an overturned jeep. Big Boss 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, Big Boss rode Diamond Horse over to Afghanistan, accompanied by Ocelot on another, brown horse, in order to rescue Kazuhira Miller, who had been ambushed by Soviet forces. Big Boss then went ahead with Diamond Horse as his only partner. After rescuing Miller, Diamond Horse was presumably airlifted back to Mother Base.

Behind the scenes
Diamond Horse first appeared in the GDC 2013 reveal trailer for Metal Gear Solid V: The Phantom Pain, although the horse's role as a character was earlier hinted at with several tests of the Fox Engine. The horse later appeared in both E3 2013 trailers, as well as the E3 2014 demo and Gamescom 2014 demo. The TGS 2014 demo confirmed that the player can use the horse outside of the Afghanistan mission and presumably the Hospital mission and is part of the buddy system in the game, and also confirmed that the horse's name is "Diamond Horse" (or rather, "D. Horse")

During Episode 32 of Kojima Station, while demonstrating some new features for The Phantom Pain, Hideo Kojima stated that players can customize their D. Horse to their liking.

Although earlier sources implied that Diamond Horse can be killed, the Official PlayStation Magazine and Xbox: The Official Magazine disputed the claim, citing that Diamond Horse can't be killed from their playthrough. It was also revealed that when whistled while the player is running, Diamond Horse can sprint alongside them and allow them to board, so they wouldn't loose speed.