While he might not be a particularly realistic depiction of a lemur, he's clearly intended to be one. He has the characteristic lemur features like a long tail, large eyes, and a nimble physique.
Momo's role in the story goes beyond just being a cute companion animal. He serves as a vital part of Aang's journey, often acting as his confidante, offering moral support, and even helping him in his fight against the Fire Nation.