Yacuruna are anthropomorphic creatures who live in grandiose, underwater cities. Belief in the yacuruna is most common among the indigenous people of the Amazon. The term is derived from the Quechua language: yacu ("water") and runa ("man").

The yacuruna have variously been described as hairy, with their heads turned backwards and deformed feet; alternately, they may appear more human-like, distinguishable by the serpent they carry as a necklace and the crocodile they may ride as a canoe. They are nocturnal beasts, emerging by dark of night to roam the rainforest, searching for prey. They can communicate with other water-dwelling creatures, and it's even possible for them to exert control over other animals.

Locals believe that a yacuruna can transform itself into a river dolphin, seek out a young woman woman for its victim, and disguise himself as a handsome young man, one who uses aphrodisiacs to seduce his captor into his joining him in the depths of the river.

Though generally depicted as sexually alluring, malevolent creatures, the yacuruna are sometimes able to heal others. When summoned by a shaman, the creature can be forced to pass its abilities over to a sick person.