Final answer:
Medusa is the creature that attacks Percy, Grover, and Sally in Chapter 4. They have to find a way to defeat her without looking at her directly.
Step-by-step explanation:
The creature that attacks Percy, Grover, and Percy's mom, Sally, in Chapter 4 is Medusa, a Gorgon from Greek mythology.
The encounter takes place in the Lotus Hotel and Casino. Medusa is a dangerous mythological creature with snakes for hair.
Looking directly at her turns people to stone, so Percy and his friends have to find a way to defeat her without meeting her gaze. They eventually manage to defeat Medusa by using a reflective surface to avoid looking directly at her.