Manta

The Manta, the Behemoth of the Deep, was a creature ten times the size and strength of a dragon. It was believed that, like Deathwing, it was unstoppable. The Manta was killed by Korialstrasz one hundred years before the Second War with an arrow covered in three drops of a black liquid from a vial.

Speculation
Manta may have been a giant kraken.