According to japanese legend, the vengeful spirit of buddhist priest, incarnated in the iron rat:

Long ago, during the reign of Emperor Shirakawa (1073-1087 CE), there lived a monk named Raigō. Raigō was the abbot of Mii-dera, a monastery in Shiga prefecture at the foot of Mount Hiei, and well known for his piety.

The Emperor, having no heir, was concerned about his line of succession. One day, he approached Raigō and asked him to pray to the gods and Buddha in his place for an heir. Raigō prayed long and hard, and finally in 1074 a royal son, Prince Taruhito, was born. The grateful Emperor promised to give the abbot anything he wished in return for his prayers. Raigō asked that a splendid new ordination building be constructed at Mii-dera so he could train new priests. The Emperor gladly agreed, however Mii-dera had a powerful rival temple — Enryaku-ji, on top of Mt. Hiei — which wielded great political power as well as having a powerful army of warrior monks at its disposal. Enryaku-ji could not abide such a gift being granted to a rival temple, and so it exerted its great pressure on the Emperor. Bowing to Enryaku-ji, the Emperor reneged on his promise to Raigō.

Raigō began a hunger strike in protest of the Emperor’s broken promise, but the Emperor would not, or could not, go against Enryaku-ji’s will. On the 100th day of his hunger strike, Raigō passed away, his heart full of rage towards the unfaithful Emperor and the rival monastery of Enryaku-ji. So great was the hatred in Raigō’s heart when he died that he transformed into an onryō, a ghost driven by pure vengeance. Shortly after Raigō’s death, a ghostly vision of the abbot was seen hovering near young Prince Taruhito’s bed. A few days later the young prince died, leaving the Emperor heir-less once again. But Raigo’s vengeance did not end there.

Raigō’s twisted spirit transformed into a gigantic rat. Its body was as hard as stone and its teeth and claws as strong as iron. The monstrous spirit, Tesso as it came to be called, summoned a massive army of rats which poured through Kyoto, up Mt. Hiei, and arrived at Enryaku-ji. There, the rat wreaked Raigō’s vengeance upon the monks. The army of rats poured through the monastery complex, chewing through the walls and doors, tearing up the roofs and floors, and attacking the monks. They devoured Enryaku-ji’s precious sutras, scrolls, and books, eating and despoiling everything they found — they even ate the precious statues of the Buddha.

Nothing could stop Tesso and the army of rats until finally a shrine was built at Mii-dera to appease Raigō’s spirit, and Raigō’s shrine still stands at Mii-dera today. An interesting footnote to the story: while Buddhist buildings are typically built facing the east, Raigo’s shrine is built facing the north. It points to the top of Mt. Hiei, directy at Enryaku-ji, the target of his rage.