The hybrid offspring of a Manta Ray and a Shark.
The term is a portmanteau of Manta Ray and Ray Donovan, the latter being a fictional TV show character that was a loan shark amongst other than things.
A new species of fish, the pelagic tropical ray feeds on plankton and smaller fish; usually harmless but its size make it dangerous if harpooned or intoxicated. Often mistaken for being of Judaic descent, the female Manda Ray is characterized by its large mammary glands and little body.
"Who is that new fish shuffling through the ball?" asked the lobster. "Oh her, that's the Manda Ray," said the sea otter.