Yami Shibai 9

Synopsis

A salaryman named Takayuki comes home from work to his son Shōta riding an old rocking horse; it was his when he was a boy, and his sister brought it from their parents' house. Takayuki remembers he and his sister often fought over it as kids. That night, he hears the horse rocking in the living room, but the room is vacant. He turns to leave, but is startled when the horse rocks on its own. He calls his sister and asks about the horse, insisting that they shared it as kids. However, because they're 10 years apart, she would've been too old to have played with it; she suggests it was their neighbor Miki. The next morning, Takayuki is watching Shouta ride the horse, when Shouta abruptly approaches him and says that "she" said it wasn't Miki, and that he's forgotten a terrible thing he did to "her"; this unsettles Takayuki - he'd never mentioned Miki. Shouta warns him to remember "her" or else... Then starts laughing loudly. The rocking horse begins rocking, and a giggling shadow appears upon it. Who was "she"? And what did he do?

(Source: Wikipedia)