Héyú (pronounced huh-yee) is a mutated saltwater crocodile.
On March 11th, 2011 in Japan, a tsunami caused by an earthquake triggered the explosion of the Fukushima Daini Nuclear Power Plant to explode, blasting the area with nuclear radiation. A group of zooplankton in the nearby water picked up the radiation, grouping into a large conglomerate of nuclear microorganisms. The large black mass traveled through the ocean to northern in Australia, absorbing small creatures as it went. When it got to Australia, it found a massive saltwater crocodile, which it then promptly smothered and corrupted. Mutating it's DNA, the tar-like substance turned the crocodile into a kaiju.
Return of the Goliath