The IGN Wiki states that D-Dog can be found during missions 3, 4 and 5. However, I am up to mission 12, and I still have not encountered him.
I have tried returning to the mentioned locations, in free roam mode, but I still do not encounter him. I am not confident I have searched correctly, and if I have gone at the correct time of day. I also tried repeating the missions, but still, no luck.
Am I doing something wrong, is there any other way to find D-Dog, or have I lost the ability to find him completely?
Best Answer
As is mentioned, in the comments;
You can find D-Dog by traveling to the starting locations before actually starting the mission. While it might involve a small amount of luck, D-Dog should show up as you approach the mission start zones for missions 3, 4 or 5.
To help in locating him, listen out for his yelping.