Clazmonian Sow

 The Clazmonian sow, which used to be called Phaea, was a savage and formidable wild beast, by no means an enemy to be laughed at. The Clazmonian Sow was one of the monsters whose mother was Echidna, who was called mother of monsters. Others relate that Phaea was a woman, a robber full of cruelty and lust, that lived in Crommyon, and had the name of Sow given her from the foulness of her life and manners, and afterwards was killed by Theseus.

The Last Olympian
Percy had to face the Sow right after he defeated Hyperion when Kronos and his army sent it in to weaken the demigods while his army killed them and tried to capture Olympus. He said that it was the boar's really angry girlfriend. Percy took a grappling hook and rope from an Athena's cabin camper and swung it around the Sow's leg as it flew away, which left him hanging dangerously onto the rope. Blackjack the Pegasus came and let Percy ride on his back so that Percy could pursue the Sow. Percy attached the rope on the Sow's leg to a Hermes statue and then activated Plan Twenty-Three which made the statue come to life and attack the Sow. Percy then did the same thing with some Lion Statues. Then finally, with the statues' help, Percy defeated the Sow, thus making him the second person to kill the Sow. He says he hoped it got to meet the "boar of its dreams" down in Tartarus.

Appearance
The Sow was described to be similar to a huge pig with wings, with a cry that was similar to the Erymanthian Boar Percy encountered in The Titan's Curse. Her wings and skin were pink like a flamingo's, and she was able to rip up entire forests with her hooves and destroy wildlife with her noxious breath.

Trivia

 * Annabeth states that no hero has ever been able to defeat the Clazmonian sow, but Theseus was able to kill the sow and its owner Phaia, on his way to Athens. However, she said "As far as I know, no hero has ever beaten it," so perhaps she didn't hear the legend of Theseus defeating the sow.
 * The fact that Percy was able to kill it may have been a reference to the fact that Theseus, who was also a son of Poseidon, was able to beat it.