Unless your character IS the monster, being afraid of monsters within a roleplaying game is almost something of a given. Especially since defeating the monster is typically the point of the game. But if your character's greatest fear is BECOMING the monster, this fear can be viewed as an excellent motivator for them to choose a path within the game's storyline that will hopefully lead them away from becoming the monster or villain.
For example, let's say your character is a wizard who is obsessed with necromancy and the dangerous knowledge that comes with the topic of study. In this case, maybe they are deathly afraid of going so far that they turn into a Lich. So, they instead decide to do everything in their power to fight against that eventual possibility. This fear can be viewed as their "greatest fear."
While the fear of monsters or becoming the monster can be seen as a rational fear, a character may, however, have a "greatest fear" which is derived the irrational variety of fears.
Perhaps your character is a Heavy Gunner (Warhammer 40,000) and seemingly isn't afraid of anything on the battlefield. How would they react to the world around them if they have an irrationally "great fear" of purple butterflies (which are typically quite harmless, FYI). And how would they react if their squad had to execute their next mission through a field populated by millions of purple butterflies?
Irrational fears are oftentimes the hardest for other characters to comprehend, while at the same time being a very fun way to play your character.
Fear can be a wonderful tool. But discovering your character's greatest fear and endeavoring to understand why it is their greatest fear can be a fun exercise in understanding your character's mental health and stability. These fears can also invite numerous opportunities for you (the player) to more deeply inhabit your character while also providing you new ways to roleplay with your fellow adventurers. If a particular fear works for your character and their backstory, don't be afraid to roleplay that fear to its fullest extent . . . no matter how rational or irrational it may be.