An Akaei is an extremely large sea-monster from Japanese folklore that resembled a gigantic stingray, so large that they could easily reach 12 kilometers in length.
They were infamous for rising from the ocean depths and becoming mistaken for islands by unwary sailors, only to sink back down into the depths after being discovered, they were not malicious by nature but their sheer size made them extremely dangerous, since when they did move they created violent whirlpools that could easily sink ships.