Hippocampus is the scientific name for the actual seahorse (and part of the brain named after it). So eventhough you had the mythological version in mind, what you typed is technically a correct answer and should have been accepted.
And then I couldn't remember the English name even though it's a direct translation from my language.
Caught between the Scylla and Charibdes,
Hypnotized by you if I should linger,
Staring at the ring around your finger.
Thanks for the point, Sting!
Could you please accept a few more spellings? :)
Percy was all I could think of when it came to sons of Poseidon, too. Well, Tyson too but that was also obviously not the answer.
For the deepest point on the seabed where the clue was ___ Deep, all my brain wanted to supply was Helm's. Doh.