(this is a good chunk of an idea, but feel free to change anything you want)

Plot: Your main character is on a road trip with their best friend, and stops at a rest stop to go to the bathroom. Their best friend stays in the car while they do, on their phone. When they get back, the best friend suggests they go on a walk because their legs are hurting, and the main character agrees. The best friend leads them on a small path deep into the forest, and then turns around and--plot twist--betrays the main character, turning out to be a strange, mythical cryptid impersonating the best friend. the cryptid chases after the main character and...

Characters: [Main character] [Best friend (could also be a romantic partner)]