A horse, an assassin, a pup, and a mech. Big Boss keeps good company.
Diamond Horse (or D-Horse if you’re fancy) is the first buddy you’ll acquire in Metal Gear Solid V: The Phantom Pain. She is loyal and fast, can get you from point A to point B with little to no issues, is upgradeable (by way of armor) and is just pretty to look at.
Unlocking D-Horse in MGSV is as simple as just completing that insane prologue at the beginning of the game. Afterwards, Ocelot will come rescue you on horseback and some time will pass, eventually leading to you sitting on a beautiful white mare. After Ocelot finishes explaining everything in Troy Baker’s soothing voice, you’re free to go anywhere you want and the horse is yours.
Use D-Horse for stealth (by hanging on the side), moving quickly while shooting, an easier means to traverse mountains, or as practice for dropping supply drops on soldiers. You can also have her poop on the road to cause enemy vehicles to lose control (we shit you not). Whatever purpose you choose for her, D-Horse is an invaluable addition to any soldier.