Kidomaru - 鬼童丸 - "Son of the oni king."
• About this yokai: The abandoned Son of the Oni king: Shuten-doji. - [if you haven't read that story yet, go do so before reading this!] - the story goes a little like this:
Kidomaru was born to one of the many maidens of which his fathers gang had kidnapped. He was Born with a full set of hair, very sharp teeth & ridiculous strength. Shortly after his birth, his mother lost her mind from the stress of the kidnapping. So, Kidomaru was sent to a temple on Mt. Hiei, The monks must have grown tired of him too, as he was soon expelled for being such an ill-behaved, violent little brat!!By the time he was 7, Kidomaru had run away to live in a cave, father like son, he soon became a bandit, stealing whatever he could from travellers to survive. - As he grew up mostly feral, he ate wild monkeys and boar which he hunted himself. - Kidomaru held a deep seeded grudge against the man who killed his father and decided to plot and scheme, swearing revenge against Minamoto no Yorimitsu.One day, Yorimitsu decided to visit his brother, to his surprise his brother had locked kidomaru in his washroom. - Yorimitsu decided to stay the night to make sure nothing bad happened. - Kidomaru very easily broke out, and went to spy on the men. - Noticing this: Yorimitsu loudly annouced "tomorrow, we will make a visit to the Kurama temple." - Kidomaru, hearing this and egarly wanting revenge, left early. He had decided to ambush Yorimistu & his men in a pasture near Ichiharano.As there was no place to hide other than in plain sight: Kidomaru waited by the roadside, dressed in the skin of a freshly slaughtered bull. Unfortunately for Kidomaru, Yorimitsu wasn't stupid, he sensed danger and ordered his men to shoot every bull in sight! - But before the arrows could fly, Kidomaru charged the men. However, Yorimitsu's blade was too fast, Kidomaru was slaughtered faster than the cow he had dressed as...
• History: Sadly not much else was written about Kidomaru; His story is one of the 750+ Heian period short Myths compiled in "Kokon Chomonjū" [古今著聞集, 1254] in short, its a spin-off of the shuten-doji story. - However, despite his obscurity, hundreds of years after the fact, Kidomaru was a popular subject of prints appearing quite widely; Notably he is featured in Toriyama sekiens Hyakki Yagyō 
Kidomaru is also featured in a number of Ukiyo-e prints where he is usually featured having a power contest with famous magic weilding bandit, Hakamadare Yasusuke. (another rival of Yorimitsus from the time, famous for his ability to use magic to summon gigantic snakes!)
[Battle of magic, Utagawa Yoshitsuya. 1860s: Source]
• One could say that kidomaru is the polar opposite of the folkhero kintaro. (another wild child, who grows up to become one of Yorimitsus loyal retainers. Unlike kidomaru, kintaro had an adoptive mother.)
• See also: Kintaro, ShutenDoji, Ibaraki Doji, Kintaro.