I'm trying to sort out how this could have happened. My best guess would be that both dogs were checked in at the same time and the baggage tags got switched? UA (among other airlines) rescans all tags as things go on the plane, you'd think they'd be double sure to cross check PETS, of all things. Are the kennels not scanned?

Of course, the other obvious question is did the dog earn miles flown or only original route credit?