Most people who play fallout four or who are new to the game think that dogmeat is new to the game even though he is featured in fallout three you can find him in a dump name of which i can not remember so don't think that he is not an new addition to the game.The makers of fallout three even took a precaution to make a perk where dogmeat can have puppies which comes in handy is he ever dies and you can find them at vault 101 which is where you come out and see the wasteland for the first time in the game.
If you look at the dogmeat from Fallout 3 and from Fallout 4, visually it is a different dog in each case. Even the Puppies perk is a replacement dog and could be considered Dogmeat Jr.
Also, considering the timeline between the two games it would be almost certainly that 3's Dogmeat would have died by now from old age. Sakaratte - Talk to the catmin 18:48, June 16, 2017 (UTC)