There are three of them and they came alive after Larson and Lara disturbed the gate. They flunged Larson across the courtyard but the fall did not kill him. Lara had to destroy them to advance to the Colosseum.
Each serpent does not move from the door of the gate, showing their loyalty to protecting the Philosopher's Stone. They can move as shown in the cut scene before Lara fights them but choose not to during the battle. Each serpent can shoot fireballs. You can see the fireball brewing in a serpents mouth before they shoot it, giving you time to prepare to manoeuvre. If Lara is hit by a fireball, she will be set on fire and will die in a few seconds if she had a full bar of health. If set on fire, Lara must run to a nearby pool of water quickly before the flames consume her. The serpents will also lash out if Lara gets too close.
By jumping sideways while shooting at them you can avoid the flame bolts. Even when they recharge, they are very weak at that point. Shotgun wideshot ammo tends to be very effective against them.