According to Javanese mythology, the creature is believed to be kept by a person practicing pesugihan tuyul black magic. Pesugihan is derived from Javanese word sugih means "rich". It is a kind of magic to help people to become rich instantly, but in exchange he or she must perform some rituals or offering tumbal (sacrifice something) to keep the tuyul happy. According to Javanese myth, tuyul can be kept for financial gain, but in exchange a female member of the family must allow it to breastfeed from her, sucking blood instead of milk.

Appearance

People normally associate the appearance of a toyol with that of a small baby, frequently that of a newborn baby walking naked with a big head, small hands, clouded eyes and usually greenish skin. More accurately, it resembles a goblin or grey alien. It can be seen without the use of magic, though they are unlikely to be spotted casually.

Those who did claim to encounter it described its actual appearance to be childlike (a toddler) with green/gray skin, bald, big red (alien-like) eyes, pointy ears and rows of sharp teeth, and sometimes reported with hairs, like a monkey. Its behavior is more animal-like. It tends to climb on rooftops either to play or prior to entering houses.

http://theunexplainedmysteries.com/Toyol-Spirit.html