Metal Gear Solid V: The Phantom Pain Message Board  

Did the 'capture Russian interpreter' sideop and he ran up to me right after I got off the gunship at the very start. Didn't see his mom. Just wanna give you guys a heads up, you might miss it, though I think it's random and DDog does bark loud enough to catch anyone's attention. Oh, and the MB staff will 'thank you for saving that cute little puppy'. XD

Can confirm that it's random as i found DDog when i was on my way to the barrack saving the "bionic professor" (don' remember the name of the mission). And he does bark really loud so you shouldn't miss him (if your not blasting some "Greatest 80 Mix" to loud).
He haven't grown large for me yet so I might have to do some missions (or smoke some cigars).

Yeah I got him on the same mission when I went in to grab the Russian Interpreter . Just at the LZ. Fulton recovery that pup

I got him on Episode 1, I ran him down with D-Horse on accident(he was in the bushes I didn't see him!)