Rocks are extremely common throughout the island, and may be picked up from the ground and carried by the player.
A rock being held, ready for throwing
They may then be brandished by right-clicking or thrown by aiming the cross-hairs and and holding down the left mouse button.
Throwing rocks is a good way to distract the Creature - either by throwing directly at it or by causing a distraction by throwing elsewhere. In either case, a brief distraction can give the player time to start an escape.