If you have never seen the movie UP go watch it now. Are you done crying?
Here’s my question, where did Dug the dog come from?
This is my best guess. Dug is a Golden Retriever. As far as I can tell the rest of the dogs in Muntz’ pack are Rottweilers, Bulldogs, and Alpha who is a Doberman Pinscher. These dog breeds don’t always have the best reputation and are very hard looking dogs. Muntz has obviously bread them carefully for intelligence over multiple generations. They can fly airplanes and cook.
On the other hand Dug is shown as being if not dumb then gullible. In the short included on the UP DVD, called Dug’s Special Mission, which takes place just before he meets Russell and Carl, he is bullied by the other dogs, especially Alpha. They blame Dug for another failed attempt to catch the bird and with taunts and threats send Dug running from the airship. They also wish Dug a happy birthday. Dug is obviously not a solid member of the pack and is unlikely to have been bred by Muntz.
It’s shown that Muntz loves dogs. In his obsession he will gladly kill anyone but I doubt that Muntz would kill a dog. Muntz surely isn’t the only person to travel with his dogs so somewhere along the line he kills some poor adventurer, finds Dug’s mother (probably already pregnant), and brings her back to the ship. In a tightknit, pseudo-militaristic pack, I can’t see that going well. Muntz could have ordered the pack to take her in but that doesn’t mean they’d be nice about it.
Whatever happened after that was dramatic enough that Alpha, who hates Dug, remembers his exact birthday. In Dug’s Special Mission he says his birthday wish is for a new master which implies that Muntz is either unaware or complacent in Dug’s poor treatment by Alpha and the rest of the pack. There is no evidence of Dug’s mother or of any littermates he might have had, suggesting he is possibly the only survivor, either chosen by Muntz or Muntz stopping whatever event was dramatic enough to make his birthdate memorable.