Skeleton

The Skeletons were monsters who were formed from ancient dinosaur teeth by Atlas.

History
While at New Mexico these Skeleton Monsters attacked Percy, Thalia, Zoe Nightshade, and Grover and Bianca di Angelo. When Bianca stabbed him with her hunters knife, they erupted into flames leaving a small pile of ash behind. She was only able to destroy it because she was a child of Hades.

The group, minus Bianca, encountered more skeletons who tried to capture them but failed thanks to Rachel Elizabeth Dare who saves Percy by confusing the skeletons, allowing Percy and his friends to escape and thanks to the giant angel statues who give the four of them a ride to San Francisco. Later, While at camp Percy figures out that Nico's father was Hades, after the skeletal warriors appear and Nico unintentionally splits the earth to swallow them up. Nico tells Percy that he hates him for not saving his sister and runs away.