Momos are monkeys that wears a fishbowl or a glass ball that astronauts or divers wear when they are in the water or in space. Momos wear those for no reason. It can be found at the Citadel Entrance.
It is only at Citadel Entrance. Battle the 3-moga team and you can catch Momo. It is level 20 or 25 at that time.