There are all kinds of stories as to how Kintarō was born, from being born in the forest after his mother (a princess) escaped fighting between his father and uncle, to one story where his mother is Yama-uba, a mountain crone, impregnated by a clap of lightning sent by a red dragon. He was said to be extremely strong even as a very small child – where he ran around naked, except for a bib around his neck with the kanji for “gold.” He was said to be very close to animals. They did his bidding, let him ride them, and possibly even taught him their language. From the time he was a small child he was fighting monsters and helping local workers.
When Kintarō grew up, he met samurai Minamoto no Yorimitsu, who was very impressed by his strength, and so made him a personal retainer. Kintarō was going by the name Sakata no Kintoki at this time, after Mt. Kintoki where he was raised. Kintarō trained in Kyoto, learning martial arts under Yorimitsu, and eventually became the leader of Yorimitsu’s Shitennō or “four braves.” Eventually he brought his mother to Kyoto as well.
Today in Japan Kintarō is a very popular figure, with his face on art, manga, and even candy. (Actually, the candy’s been around for centuries). If you’re interested in Kintarō, there is an anime called Golden Boy that is based on the legend.