Merperson

Merpeople are monsters that live and work with Poseidon in Atlantis.

Description
They appear human, but instead of feet they have fish-like tails. They have shark-like teeth and a green or blue skin color. Poseidon's son and heir, Triton, appears to be an alternate form of a mermaid with two tails rather than one. They are sentient beings found in many myths other than Greek.

The Last Olympian
Percy wakes up in Poseidon's palace underneath the sea. There he sees many merpeople wearing battle armor ready to battle the forces of Oceanus and some making underwater graves for fallen merpeople.

He also meets his paternal, immortal, half-brother, Triton, a merman with two fish tails instead of one and his stepmother Amphitrite.