If Grover were confronted by Chernabog, the Mean Scary Creature from Disney's Fantasia, he would likely react with surprise, fear, and maybe a touch of confusion. Grover is known for being a lovable, furry monster in the Sesame Street universe, and encountering a dark and menacing figure like Chernabog would be quite out of his ordinary experience. His typical reaction might involve humorous attempts to navigate the situation or seek help from his friends on Sesame Street.