Because he is either a bonobo or a Barbary macaque, Curious George the monkey probably doesn’t have a tail. His creators, H.A. and Margret Reys, are both dead, so they never answered this important question.
Most monkeys have tails that can be grasped, but apes don’t have tails at all. Because of this, people who know the story are probably confused by the fact that he has no tail. When Curious George was made in the 1930s, monkeys were often used to describe small apes and even chimpanzees. Even though this is wrong from a taxonomic point of view, it was acceptable at the time. Because of this, George may look most like a bonobo, which is a small chimpanzee.
Alternatively, he could be a Barbary macaque. Macaques are a type of monkey from Africa, but this species doesn’t have a tail. But George doesn’t look like a Barbary macaque, so it’s most likely that he’s a bonobo who’s been called a monkey by mistake.