D-Dog is one of the best buddies available in Metal Gear Solid V. More information about all the buddies can be found in a separate chapter of this guide. Below you will find a short manual on how to find and take the dog in, so that he could start helping you as early in the game as possible.
After visiting the Mother Base for the first time, select the fourth main mission from the list - C2W (you can do that before starting the third mission). Immediately after starting that mission check the area near the landing zone. In the place shown on the picture above you should find a puppy.
Warning - A dog may also appear in other places and you may find him even before the fourth main mission. While you are exploring the world keep listening and if you will hear any bark just look around if you want to find its source.
You must use the Fulton to send the puppy to the Mother Base. After completing the mission Ocelot should ask the hero if he wants to return to the Mother Base. A short cutscene with the puppy will occur there. Now you must wait for the dog to become larger and be "promoted" to the status of a buddy.
Important -if the puppy is nowhere to be found near the landing zone, then try to start the mission from the beginning or select a different time of day when repeating it.