This is a frilled shark.
It is, say the news reports of a recent sighting and catch of one of these creatures, rarely seen. This is because it lives in the deepest darkest depths of the deepest darkest bits of the deep dark ocean.
Half-shark half-eel, the frilled shark is a cannibalistic bottom-feeder that eats other sharks.
It has evolved very little since prehistoric times.
"Rarely seen", eh? These people clearly don't spend a lot of time in the blogosphere.