At six in the morning on September 25, 1843, a group of government officials and laborers were working on the poldering of Lake Imba when a sudden rainstorm struck them, heralding the arrival of a terrible monster.

From the bubbling waters emerged a strange creature, over 15 feet tall, monkey-like and covered with black fur, with a snub nose and long sharp claws. It sat unmoving atop a rock for a while, staring at the horrified witnesses, before it called forth a flash of lightning, killing over a dozen men at once and severely injuring the rest, a few lived to tell the tale but fell ill after the incident. The monster disappeared, never to be seen again.

The Lake Imba Monster, also known as the kaiju of Imbanuma, is a local enigma as this is the only instance of its appearance, unlike other yōkai creatures. While some believe the legend to be a tall tale, perhaps inspired by a seal that strayed inland and confused the locals, there is also a theory suggesting that it was made up as a protest against the soldering work.

In 2020, the Yachiyo Local History Museum created a 1:4 scale model of the Lake Imba Monster for a new exhibit, based faithfully on the depictions recorded in three contemporary texts. Its appearance seems to solidify the seal theory, but also feels like a realistic cryptid, perhaps still lurking deep in the marshes of Imba.