The hardest animals to draw are definitely monkeys.
According to a theory which says that humen are harder to draw than animals, they say it's because we see them as individual persons. Animals are perceived by specieses, and when I draw animals, I put together all the main features that animal species has. Horses are horses, if I drew a horse, people wouldn't ask have I ridden that horse or which carrot flavour does it prefer, unlike if I painted a human, they'd ask me who is the model.
So, I might see monkeys as individually as I see my folks. Which naturally means that when I manage to paint one, I'll give it a personal name, something else than Spider Monkey, or Just Monkey.