Telekhine

Telekhines (or Telchines or Telkhines) were mysterious magician-smiths and sea demons native to the islands of Keos and Rhodes. They were the children of Thalassa and Pontus.

History
They invented the art of metal-working and were said to have crafted the sickle Kronos used to castrate his father Uranus and later a magical trident for Poseidon which the god used to level mountains into the sea and form the islands. Their malignant use of magic later angered Zeus, who cast them beneath the sea or into the very depths of Tartarus. Telkhines were sometimes described with the heads of dogs and fish-flippers for hands. They're also known as "fish children".

Appearance
Telekhines have faces like dogs, with black snouts, brown eyes and pointy ears. Their bodies are sleek and black like seals. They have stubby legs that were half flippers and half foot, and humanlike hands with sharp claws. Percy described that if you blended together a Dobermann pinscher, a child and a sea lion, they'll look a bit like a Telekhine. Fully grown adults can be up to two and a half metres tall. Telekhines bark and growl, and they waddle instead of walking.

The Battle of the Labyrinth
They made weapons for the Titans and used Hephaestus' forge, under Mount Saint Helens. Percy spied on them, and burst out while the younger Telekhines were watching a documentary. It is learnt that they are on the side of Kronos, holding a grudge against Zeus and the other Olympian gods. They had to leave the forge when Percy caused the volcano to blow and he almost dies. Later some Telekhines were seen with Ethan, carrying Kronos's scythe, helping to resurrect the Titan on Mount Tamalpais.

The Last Olympian
When Percy was aboard the Princess Andromeda with Beckendorf, he spared a telekhine, partly because his "Li'l Demon's Lunch box was cool". The East River Spirit reminded Percy of a telekhine.