I think of Zap as a small monkey; in fact, I often call him that. He looks like one, especially from the back, and he acts like one. However, his little paws are definitely those of a dog, not a primate. That limits his usefulness as a service animal, but it also keeps our food bill down.
I was fascinated to learn that monkeys are actually being trained as helpers for the disabled. They've always scared me a little and I could easily imagine one burning down the house, but these are so damn cute. Just look at the video. And if it didn't work out, you could always rent it out as a babysitter.