In either versions, they were served as supporting antagonists as both versions were affiliated with Luke and Kronos.
The book version of the monster is a male manticore under alias of Dr. Thorn whom like all monsters, capable to shapeshifting into a human form as means to blend in human society. Dr. Thorn used this said alias in order to infiltrated Westover Hall as a new teacher.
When Percy Jackson, Annabeth Chase, and Thalia Grace arrive at Westover Hall because Grover Underwood claims to have found two incredibly strong demigods, they are stopped by two of the teachers. Thalia uses the Mist to fool one of the teachers, but the other named Dr. Thorn is not completely convinced. During the dance the school is having, Dr. Thorn takes the two children and Percy rushes after them. When he enters the hallway, he finds Nico and Bianca di Angelo, but Dr. Thorn stabs Percy with a spike he shot from his scorpian tail and reveals himself to be a manticore. Being unable to fight him in his weakened state, Percy goes along with the manticore and tried to contact Grover using his Empathy Link. The manticore reveals that he is under orders from the General to capture the two half-bloods in order to recruit them in Kronos' army.
However as their transportation arrives, the manticore is attacked by Annabeth, Grover, and Thalia. The manticore is forced to use his original form to hold them off until Artemis and her Hunters arrive, claiming the manticore as their prey. Annabeth jumps on his back and stabs him with her knife, but it jumps off the side of the cliff and vanishes, bringing Annabeth with him as a prisoner. Artemis later questions Percy about the manticore and agrees to hunt it an the monster he claimed they would have soon. The manticore later delivers Annabeth to Luke Castellan, who uses her as bait in order to lure Artemis into a trap.
The manticore appears again in the presence of the General and Luke. He informs the two of Zoë Nightshade and her companions, currently questing for Artemis. The General belittles the manticore for his lack of results and denies him his request for a command of his own. The General then sends the manticore out of the room.
The manticore later appears again, cornering the questers on a dock. He claims that he will defeat the demigods himself in order to prove himself, but keeps them alive so they can witness Thalia killing the Ophiotaurus in order to defeat the gods. Thalia is tempted by the possibility, but refuses. The manticore then attacks and almost kills them all until Percy sends an Iris Message to camp and pleads with Mr. D for help. The manticore is then covered in vines shortly after and is turned to dust.
Unlike Dr. Thorn whom featured in the book, this version of Manticore appeared behaving much like a feral war dogs, but still loyal to it's superiors includes Luke and it's gender was unknown albeit presumed to be male. The manticore is first appears on Luke's ship the Princess Andromeda. It stops Percy Jackson, Annabeth Chase, and Tyson from escaping. After the gang escape the brig they sneak by it while it's sleeping near the door.
It appears at Polyphemus' lair during the reviving of Kronos. Percy runs to battle Kronos, but the manticore stops him. Percy and the manticore fight, but it knocks him down. It leaps at Percy, but gets hit by a flying rock. After Percy defeats Kronos, the manticore reappears and stabs Annabeth from behind. Clarisse and Grover go to fight it. Grover distracts it while Clarisse chops off it's tail. It destroys the manticore turning it to stone and crumbling to dust.