According to Wikipedia the Amemasu or the Ō-amemasu is a giant whale- or fish-like creature from Ainu folklore. Some of the largest amemasu are said to live in Lake Mashū and Lake Shikotsu in Hokkaidō, with smaller ones inhabiting lakes throughout northern areas of Honshu. The amemasu are known for capsizing boats, creating earthquakes, and causing other natural disasters.
The Amaburakosagi are young Japanese men dressed up as scary ogre- demon oni and they go house to house to terrify and scare children out of laziness and disobedience on New Years Eve. They also taunt new wives.