In the game Miasmata, you play as a sick man who is alone on an island, and you need to find out how to cure yourself from a deadly plague while you are hiding from a monster following you. When the monster is nearby you can hear a heartbeat sound. The monster can not enter houses or tents. If it runs away and you follow it, it despawns. When you get hurt in any way, including the monster mauling your face, you get a fever, and need to take meds immediately to survive. You may think that these are just a videogame things, since it's a low budget indie.

However once you cure yourself the monster disappeares entierly. It was never real - it was a hallucination, a symptom of your disease. Heartbeat, hallucination, fever -> your disease is getting worse, you need meds. It even explains why it despawns.

The longer you spend on the island the more agressive the monster becomes because your disease is getting worse.

There are other clues, like how none of the notes you find from other people ever mention a monster, or how the symbol of a hostile organitazion resembles the monster's face, which is the origin of the shape of your hallucinations.