It is possible that fictional monsters inspired by real-world parasites could include:
A creature that can control the behavior of its hosts using chemical signals, similar to the hairworms or parasitic wasps. In the movie or video game, this monster could be depicted as a small, insect-like creature that can infiltrate the bodies of its hosts and take over their minds, causing them to act in ways that benefit the parasite.
A monster that can manipulate the behavior of its hosts by altering their memories or thoughts, similar to the Toxoplasma gondii parasite. This creature could be depicted as a powerful, otherworldly being that can enter the minds of its victims and warp their perceptions of reality.
A monster that can alter the appearance of its hosts to better blend in with their surroundings, similar to the cuckoo bird. This creature could be depicted as a shape-shifting monster that can take on the form of its hosts, allowing it to infiltrate their social groups and take advantage of their resources.
As for which of these behaviors is most disturbing to me, I would say the ability to manipulate the thoughts and memories of a host is the most disturbing. It is unsettling to think about the possibility of someone or something being able to alter one's perceptions of reality and manipulate one's thoughts and memories.
If I were designing a movie or video game monster based on the mind-controlling hairworms, I would depict it as a large, writhing creature with long, slender tendrils that it can use to infiltrate the bodies of its hosts. The monster would be able to take over the minds of its hosts, causing them to act in ways that benefit the parasite. In order to escape its control, the player character would have to find and destroy the monster's vulnerable "nucleus," located deep within its body. The monster could also have the ability to manipulate the memories of its hosts, causing them to doubt their own perceptions and making it harder for them to resist the monster's influence.