From Kanjaku, translated by Royall Tyler (Japanese Tales, 1987). Masamichi was a real person who died in 1017, during the Heian era.
Lord Minamoto no Masamichi lived south of Fourth Avenue and west of Muromachi Street in Kyoto. His son, not yet two years old, was playing by himself outside when suddenly Masamichi heard him howl, and there were loud shouts from the nurse looking after him.
Masamichi picked up his sword and ran from the other side of the house to investigate. He found not one but two identical nurses struggling for possession of the boy. Each had hold of an arm and a leg. Clearly one of them must be a fox, though he had no idea which. As he charged, brandishing his sword, one of the nurses vanished.
The other nurse and the boy collapsed on the ground. Masamichi had his servants call in a healer of proven power, and after the healer had worked his rites awhile the nurse came to. At last Masamichi was able to question her.
"I was letting the young master play a little by himself, sir, when a woman I'd never seen before came out of the house and claimed the young master was hers. In fact she tried to take the young master away, and I held on to him to stop her. When you ran up with your sword, she let go and rushed back into the house."
Masamichi was seriously frightened. He never found out whether the double had actually been a fox or some sort of angry spirit. People commented, though, that you just shouldn't let little children play by themselves.