Definition of "oni"

ONI 鬼. Generic name for devils, the representation of which in art is quite a common feature. Onis have claws, a square head with two horns, sharp teeth, and malignant eyes surmounted by big eyebrows; occasionally they wear trousers of tiger skin.

1908, Henri L. Joly, Legend in Japanese Art: A Description of Historical Episodes, Legendary Characters, Folk-lore, Myths, Religious Symbolism, Illustrated in the Arts of Old Japan, pages 263–264

Across the ocean, in Japan, there once lived curious creatures called Onis. Every Japanese boy and girl has heard of them, though one has not often been caught.

1918, William Elliot Griffis, Dutch Fairy Tales for Young Folks‎

"That's no human being playing the instrument," he thought in amazement. "It can only be an oni or some such being."

1979, Marian Ury, Tales of Times Now Past: Sixty-Two Stories from a Medieval Japanese Collection, University of California Press, page 147

The standing Shoki holds with his left hand an oni on his leg.

1992, Karl M. Schwarz, Netsuke Subjects: A Study on the Netsuke Themes with Reference to Their Interpretation and Symbolism, Böhlau Verlag Wien, page 46

This is actually a boy bishie in the form of an ogre. It's called an oni in Japanese. Onis have supernatural powers that can command the forces of nature such as wind (to create hurricanes) and lightning (to create thunderbolts).

2005, Christopher Hart, Manga Mania Shoujo: How to Draw the Charming and Romantic Characters of Japanese Comics, page 69

The oni are a diverse group of evil spirits who take on the form of humanoid creatures so that they can enjoy the pleasures and vices of the flesh.

2011, Mike Shel, “Ecology of the Oni”, in Jade Regent: The Brinewall Legacy, Paizo Publishing, page 69