As a series of anatomically correct myths, here we have the Nuckelavee. Is there a realistic way that Nuckelavee could evolve? Using Earth or near-Earth biology, how close could I get to the classic Nuckelavee? Is there a reason that a Nuckelavee couldn't evolve? To me the two heads and split spine is the major problem. You could imply that the horse head is just a glorified trunk. It's up to you.
A list of all of the Anatomically Correct questions can be found here