Terraphobia (from the Greek Téras meaning "Monster") is the fear of monsters. This phobia is most commonly found in small children and usually goes away as one ages but can carry over to adulthood as well.
It may happen when an child or an adult is watching an horror movie and then some monster pops out and may jumpscare the viewers and the Teraphobia is ocure on the child or an adult.
For a complete understating of Teraphobia check this article: https://mindsetopia.com/teraphobia/