I'm not a dog person. Living in a small apartment in San Francisco is not conducive to taking care of a large, drooling animal (though that doesn't stop a lot of people around here). In the world of Metal Gear Solid V, though, D-Dog (or "D.D.") is my constant companion.
D-Dog is the second of Metal Gear Solid V's A.I. companions, and by far the most useful. As soon as you come upon an enemy base or guard post, he'll bark and the horizon will light up like a Christmas tree with enemy positions and hostages, which is especially valuable when you're trying to rescue someone. Once you're in a facility, D.D. can bark to draw enemy attention, bite and distract them while you run up and attack from behind, or simply kill them (provided you're carrying the right equipment). When you're done, you can pat him on the head and he'll bark like a good dog (such a good dog).
In the early going, it was tough to leave behind D-Horse's mobility. Running across the desert didn't hold a lot of appeal for me - a matter that wasn't helped by the fact that the mission immediately after obtaining D.D. involved chasing down and destroying enemy vehicles. But after a few missions with D.D. at my side, it became difficult to use anyone else. I found uses for my other companions in certain situations - D.D. isn't great when fighting Skulls, for instance - but otherwise, it was hard to give up his detection and distraction abilities. If I needed mobility, I could steal a jeep using the Fulton Extraction System and simply drive to my destination. Sorry, D-Horse.
D.D.'s appeal is aided by the fact that he is one of the best realized and most adorable animal characters in a game to date. He belongs on a very short list of memorable animal companions in a game that includes Agro (Shadow of the Colossus) and Dogmeat (Fallout) - animals who feel like major characters in their own right. A lot of it is in how realistically D.D. acts and moves, and his usefulness in combat can't be understated, but there's also the fact that only Metal Gear Solid would put an eyepatch on a dog and just pass it off as something that makes logical sense. I mean, of course Big Boss's dog would have an eyepatch, right?
Of course, the eyepatch doesn't come right away. When D.D. first appears, he's a tiny puppy who will find you in the desert. If you desire, you can attach him to a Fulton Recovery System and send him to base with a plaintive yelp (you can also ignore him, but why would you ever do that?) Between missions, you can visit D.D. at Mother Base, where it's revealed that Ocelot is training him to be your companion. Come back enough, and you will find a fully-grown wolf who is ready to obey your command.
"That wolf sure grew up fast, didn't it," a guard later remarks in one of Metal Gear Solid's typical nods at the fourth wall.
Animal companions are surprisingly infrequent in video games, likely because they require a lot of extra work to be worthwhile, and because they require extra gameplay systems. Someone had to put a great deal of care and attention into modeling D.D., building up his behavior, and designing his gameplay systems. Metal Gear Solid also goes the extra mile in having D.D. appear in the chopper, usually either riding right in front of you or up front next to the pilot. I'm constantly impressed by how Metal Gear Solid V goes the extra mile with details like these.
The extra work is worth it, though, because D.D. is an instantly memorable addition to Metal Gear Solid V. By putting so much into bringing D.D. to life, Metal Gear Solid V's development team manages to forge a bond between him and the player; and in turn, deepen the player's understanding of Snake. I know that I felt guilty anytime D.D. got hurt, even knowing that D.D. was effectively invincible and would always get airlifted in a bad moment. His yip of pain was just too painful.
I don't know that every action game needs a pet; but as an animal lover, the sight of a dog or a horse at my character's side is always a welcome one. And in my opinion, D.D. is one of the best video game pets to date.