Gyuki(牛鬼) is the name given to the fearsome yokai which lives on Mt. Nejireme, said to have the face of a cow and the body of a spider. According to legend, he disguises himself as a human and when he reveals his true appearance, it is even more dreadful than his reputation would imply.

He was once a human named Umewakamaru and was born into a house of nobles. At the age of seven, after his father's death, he began serving at the temple on Mt. Hiei to pray for his father's soul and it was during this time that his mother left him there and went off on her own. Though Umewakamaru showed a great aptitude for studies and learning, his peers at the temple became jealous and injured him several times. Realizing he didn't belong there and missing his mother, he snuck out of the temple and began making his way to the capital. At Ootsu, he was tricked by a pair of yokai who claimed that his mother was ill and staying with them on Mt. Nejireme. As a result, he was eaten by the same yokai which had devoured his mother. However, through the power of his hatred and will, he was reborn as a yokai after his death - destroying the yokai which had killed his mother and himself in the process.

Source:Wikipedia