George is a rare manx monkey.So there you go - the reason he doesn't have a tail like other monkeys.
- Alternatively, George got too curious and accidentally had it lopped off.
George is actually just a child who is pretending to be a monkey.It would explain why everyone calls him a monkey when he's actually a chimpanzee. A kid wouldn't even know the difference, and probably wouldn't know to correct you if you said, "Oh, you're pretending to be a monkey". Also, when chimpanzees mature, they can also be very dangerous to others. If George is actually just a child, it would explain why he's able to run around without ever getting into trouble or taken away by the local animal services. And any particularly crazy happenings can probably be explained by a particularly overzealous bout of long imagining (on George's part).
George has some kind of muscular disorder.This is why we never see George with typical monkey strength and why the Man and the Yellow Hat brought him home with and feels okay leaving him with untrained people. George was abandoned because he was weak and couldn't be reintegrated with the rest of his troop because of it. However he was noted as being extremely curious and smart. The Man takes him home with him instead as a long term experiment seeing if raising a monkey/chimp as a human child will yield a monkey who could be treated as a human. So far the experiment is a success.