Skeleton Warriors

The Skeleton Warriors were monsters who were formed from ancient Dragon teeth.

History
In Greek mythology, Skeleton Warriors, or Spartus, were creatures that were created by planting the teeth of a dragon sacred to Ares. Once the teeth were removed from the beast when killed, one must plant them in black soil and plow the land with a breed of ox that was rumored to have extremely thick armor plating. When this was done, from every tooth in the ground, a warrior clad in full Greek armor would burst from the earth. The most effective way to kill them is to get them fighting each other, for in most cases, attacking and actually killing one would be impossible (unless you were a child of Hades, as shown by Bianca when she stabbed one and killed it). Skeletons would reform even if they are cut in half, or have parts of their body off.

The Titan's Curse
While at New Mexico, the Skeleton Monsters attacked Percy, Thalia, Zoë, Grover, and Bianca. When Bianca stabbed him with her Hunter's 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 (but they did not know at the time) yet when she shot them with arrows, nothing happened.

The group, minus Bianca, encountered more skeletons at the Hoover Dam. Percy is separated from the group after hiding in an elevator that takes him down inside the dam. He then runs into Rachel for the first time and she saves Percy by confusing the skeletons long enough for him to escape back into the elevators. Percy and his friends regroup to try and escape. Thanks to some advice from Athena (in disguise) and two giant angel statues brought to life by Zeus, the angels 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.

The Son of Neptune
Frank Zhang was given a dragon tipped spear that could summon a skeleton warrior named Gray, but he could only summon him three times. He used the first charge to kill a group of Basilisks, the second to kill a group of Laistrygonian Giants, and the third to protect a group of people from a Gryphon attack. These warriors are also known as "spartus".-Kaitherine

Appearance
The skeletons have transparent gray skin and yellow eyes. They wore gray muscle shirts, camo pants, and combat boots. They had transparent flesh that showed their bones. Their clothes would shift and look like a uniform at different places and times. They appeared wearing police garb, military outfits and security guard uniforms.