MGSV: The Complete Guide for D-Dog (spoiler free)

D-Dog is an abandoned puppy you will find near the beginning of the game he is a very useful companion for a more stealth approach to the game you can raise a bond with him, he has 5 very useful Abilities and most importantly you can pet him, in this guide there will be 3 sections where to find him and how long it takes him to grow up, his abilities and commands, and how to raise you're bond with him.

D-Dog is found in the grass by a tree, north of Spugmay Keep in the southernmost part of Afghanistan map, he will be barking and ocelot will call you about him, use the Fulton device on him and he will go back to Mother Base, once he gets to the base he will grow overtime you don't have to do anything to make him mature it just happens be patient, sometime after mission 10 when you take a chopper out of Mother Base you will get a cutscene where ocelot shows you that he is fully grown and he will jump into the helicopter and after that you can deploy him as a buddy.

When you go near an enemy camp or base he will mark hostiles, prisoners and weapons, while just walking around he will mark wild animals and plants to help build up resources and your zoo, here are all the commands he has (you need to raise you're bond with him to unlock some of these)
Wait - he stays at his current position and sits down
Call/Whistle - he will come back to you
Keep em busy - he runs over to hostiles and barks or bites them
Do it (kill enemy) - he kills the enemy closest to him that is marked
Bark - he barks to attract enemy attention
Once you have maxed out you're bond with DD you can customize his eye and coat color (via the development buddy equipment section)
D-Dog has 12,000 health and 6,000 stamina.

These are all the ways you can increase bond level with DD:
Visit DD at mother base - +0.5
Convert one of DD's markers into a permanent marker - +0.3
Calling him or telling him to wait - +0.3
Clearing a mine marked by him - +0.3
Collect a plant marked by him - +0.3
Using the keep em busy command - +0.3
Telling him to kill an enemy - +0.3
Have DD extract a person - +0.3
Telling him to bark - +0.3
Using the DD command and he reacts - +0.3
Pet D-Dog - +0.3
Look at him in first-person for 5 seconds while in the ACC - +0.5
If he is badly wounded you will lose -2 bond

And that concludes my guide on D-Dog thanks for reading and have fun with your furry companion and Enjoy MGSV!