Long ago, there was a fisherman named Tayuu Kiribatake living in the village near Lake Yogo.
One day, he followed a pleasant smell to a willow tree. Walking closer, he saw a light-weight, bright fabric hanging from one of its branches. He picked it up to take it, but when he turned around, a beautiful woman was standing behind him.
“I am from heaven. I am drawn by the beauty of Lake Yogo, and once a year I come to bathe here. Please return my hagoromo (feathered robe) to me.”
But Tayuu did not return her robe. He ran and hid it instead.
After what seemed like endless arguing, the beautiful woman gave up and became Tayuu’s wife.
But her thoughts were consumed with her desire to return to the sky, and she cried constantly. Eventually, she gave birth to a treasured boy. One day, he heard his nursemaid singing, “Your mother is a tennyo. She is a tennyo from the stars. Your mother’s hagoromo is under a thousand bunches of straw.”
The boy went looking under the straw in the back garden. And just as the song had said, he found the hagoromo.
The tennyo was wildly happy. She wrapped herself in her hagoromo and flew away back to the sky.
A monk of Kanzan-ji temple heard this story and took pity on the motherless boy. He adopted him and brought him back to his temple. The boy was the adopted child of Sugawara no Koreyoshi.
In other words, he grew up to become Sugawara no Michizane.
Taken from a website of Hagoromo Legends from Lake Yogo.
Sadly, the English Wikipedia page on Sugawara no Michizane does not include the rumors of his tennyo mother. But they are included on the Japanese Wikipedia page.