D Dog is initially found as an orphaned wolf pup by Venom Snake in Afghanistan. He’s brought back to mother base via fulton device, where Ocelot begins training the young pup to accompany Snake into the battlefield.
Eventually fully grown-and sporting an eyepatch to cover his lost eye-looking much like Big Boss and Venom Snake-D Dog acts as a scout and asset for Venom Snake on several missions, sniffing out enemies, prisoners, animals and resources.
He can also be fitted with an artificial eye, have his coat changed, and be fitted with suits and the ability to distract, stun, or kill enemy soldiers. He carries a knife in his mouth for the later tasks, which of course is very similar to the FOXHOUND logo.
Snake also has the option to pet and compliment D Dog as well.
Serving as the mascot of Diamond Dogs in a sense, and a topic of conversation among the soldiers, first about the puppy’s paws:
Mother Base soldier 1: That puppy the boss brought back is so cute.
Mother Base soldier 2: Yeah, too cute.
Mother Base soldier 1: Man, I wanna pet him.
Mother Base soldier 2: Yeah, just once.
Mother Base soldier 1: I wanna touch his paw pads.
Mother Base soldier 2: Yeah, ju- wait, what?
Mother Base soldier 1: You don’t… wanna touch ’em too?
Mother Base soldier 2: No, that, uhh… never really occurred to me.
Mother Base soldier 1: You don’t know the awesomeness of a puppy’s paw pads? Mother Base soldier 2: I guess not… But yeah, I bet they would feel nice.
Mother Base soldier 1: Right?
And then later in a debate about if he’s a dog or a wolf:
Mother Base soldier 1: (heh) That dog’s all grown up now.
Mother Base soldier 2: You mean DD?
Mother Base soldier 1: Yeah. Didn’t seem to take that long, did it?
Mother Base soldier 2: You think he’s really even a dog?
Mother Base soldier 1: Mm? Of course he’s a dog. What’s wrong with you?
Mother Base soldier 2: Well, I mean he just looks more like a wolf to me.
Mother Base soldier 1: That’s crazy talk! He’s a dog, you hear me, a dog!
Mother Base soldier 2: So you think he’s a dog, I get it. Keep a freakin’ lid on it. What’s important is that he’s one of us, regardless if he’s a dog or a wolf.
Mother Base soldier 1: He is a dog, though.
Mother Base soldier 2: (sigh) …Why do I even bother?
This debate also is part of Huey Emmerich’s justification for his criticism of Diamond Dogs when he’s caught repairing Selanthropus and mutating the Vocal cord parasite virus (as well as being partially responsible for XOF’s destruction of Mother Base years earlier)
Huey: What do you think you’re doing?! Go ahead and execute me, it’ll be murder in the eyes of the world! You’ve lost your minds! Don’t you get it?! You’re seeing phantoms… just look at that dog! No, you named him D-Dog, but it’s obvious, anyone could see that’s a WOLF! Because you’re all a bunch of wild dogs! You wanted to believe he was too, to feel some connection, to fight your loneliness! You wanted something to cling to, to prove you deserved to be alive! You wanted to forget the death, your sins, so you cling on to dogs, or wolves, or even Big Boss… the Boss is the same, isn’t he? Every one of you is alone, that’s why you suspect your own! I know… because I’d do the same. I’m one of you too! Alone…
A D Dog plush can also be used in the online matches as a distraction (Although he’s got both eyes).
Although D Dog’s fate is unknown, the Metal Gear saga’s fascination with Wolves does not end at this point in the timeline. Solid Snake-who raises Huskies-50 of them and races with them- has to deal with Liquid’s henchwoman, Sniper wolf, who is often accompanied by Wolves as well-although those wolves will be friendly if you have her hankerchief.
He also deals with her successor, Crying Wolf-part of the Beauty and the Beast unit-in pretty much the exact same area when he returns to Shadow Moses to stop Liquid from taking Metal Gear Rex’s railgun.
A rebuilt Wolf robot named Blade Wolf likewise accompanies Raiden in Metal Gear Rising.