merhorse

plural merhorses

1. rareA seahorse.

   […] super-otter, merhorse (a kind of sea horse), marine saurian and father-of-all-the-turtles.

   1988, DeWitt Wallace, Lila Bell Acheson Wallace, The Reader's digest

   1 more example

   But there seems enough evidence to suggest that the merhorse may possibly be a distinct and genuine species. The creature is described as […] but there was no reappearance. A local expert was of the opinion that it was a 'real sea-horse'.

   1998, Nigel Suckling, Bob Eggleton, The book of sea monsters

2. A mythical sea monster with a horse-like head, mane, and large eyes.