Akago (yokai) (赤子 (妖怪))
"Akago" is yokai (specter) who is said to live in Nagano Prefecture and Yamato Province (present-day Nara Prefecture). Another view is that Akago is yokai which appeared in "Buson Yokai Emaki" (Picture Scrolls with Monstrous Beings) created by a Haiku poet, Buson YOSA. Each view has a different tradition.
Summary
Akago in Nagano Prefecture
It is said that it lives in Kizaki-ko Lake, one of Nishina Sanko (Nishina three lakes) in Omachi City. It looks like a human being aged 11 or 12 in appearance, but as the name indicates, it has a red body like a newly-born baby and hair like a Shojo (a kind of Japanese sea spirit with red face and hair and a fondness for alcohol). It is said that fishermen sometimes see it hiding himself under the water, but it does not harm people.
Akago in Yamato Province
It is described in picture scrolls of specters in the Meiji period called "Bakemono Emaki" (Picture Scrolls with Monstrous Beings) (writer unknown) with a title 'Akago no kai' (mystery of Akago). Once upon a time, there was a house called bakemono yashiki (haunted house) which nobody visited. A swordsman stayed at the house to identify what it really was. In the middle of the night, he heard a strange sound like someone was dancing on the other side of shoji sliding doors. When he peeked into the next room, he found someone like newly-born baby dancing and its number was multiplying enormously until it reached several hundred in the end. He tried to draw his sword to kill them, but he was frozen with horror. He stood paralyzed all that night and then, at the crack of dawn, all Akago (babies) vanished.
Akago in "Buson Yokai Emaki"
It is described with the title of 'Akago no kai.'
One day, when a priest stayed at a zashiki (Japanese style guest room with tatami flooring) called Ogasawara, he heard a bustling sound like a lot of people were dancing in the next room in the middle of night. When he peeked into the next room, he saw thousands of naked babies were dancing. Although they made a terrible noise all night, at the crack of dawn, they vanished.
The title is same as "Bakemono Emaki" and the content is very similar to it, but their association is unclear.