Peleus

Peleus (Πηλεύς in Greek) is the dragon that guards and protects the Golden Fleece that hangs on Thalia's tree. He is probably named after the king Peleus, father of Achilles, to whose wedding with Thetis, Eris was not invited to, and where she threw the golden apple that started the Trojan War.

Description
At first a Baby dragon, Peleus soon grew large enough to encircle Thalia's Tree. Peleus is a snake-headed dragon with purple scales and yellow eyes.

Peleus is a tame and domesticated dragon that appears to be loyal and even fond on the campers, as Annabeth have been show petting him in the past with no ill response from the dragon. However, Peleus seems to be fiercely dedicated to his duty given that he's always around the Golden Fleece, and Percy once feared that he would attack Rachel Elizabeth Dare on sight, implying that Peleus is violent toward strangers. However, given that he didn't attacked her, it's safe to assume that Peleus is intelligent enough to distinguish an ally from a foe.

The Sea of Monsters
Since Percy, Grover, Annabeth, and Clarisse brought back the Golden Fleece from Polyphemus' Island to put on Thalia's tree, they had to find a suitable guardian. Peleus, a baby dragon at the time, was chosen. Chiron and Argus fed and looked after him until he was able to care for himself and the Fleece.

The Last Olympian
When Rachel forced Blackjack to fly her to Camp Half-Blood, Percy was worried that she might get attacked by Peleus since she was a mortal. However, the dragon didn't attack her, possibly under orders from the gods or he knew of Rachel's importance as the future Oracle.

The Lost Hero
When Annabeth is giving Piper a tour of the camp, Piper sees the Golden Fleece and thinks it is a decoration or a forgery of the original (being that she is in a Greek camp) until she sees Peleus, realizing it is the real thing.

Trivia

 * Peleus may be named after the father of Achilles.
 * Peleus was the son of Aeacus, king of the island of Aegina.