There are two ways I would look at it.
1. (rather predictably being into evolutionary biology) - what would evolve in that environment and how does it survive? What's it's ecology? For example, studies in Loch Ness show that there isn't enough biomass to support a species as large as a Loch Ness monster. There are lots of creatures in fantasy stories that are completely implausible from an ecological point of view and I'm left wondering how it evolved and what it would eat if the MC wasn't around to be chased with threat of being eaten. (This isn't enough of a problem to stop me reading on though.)
Most people don't notice this sort of thing, but if you are worldbuilding it's definitely something to consider. Instead of "what creatures could I invent/randomly place here" you could start with "how does the food web work? What do the apex predators look like? Are there creatures who eat smaller things but get eaten by bigger things?" And also "what characteristics would creatures have to survive in this environment?" which leads to different habitats - forest monsters, swamp monsters, sea monsters, monsters that roam open countryside, etc. Also, life as an apex predator is precarious and most ecosystems support only one. You can get situations like the Neandertals/bears situation in ice age Europe - sometimes bears ate Neandertals and sometimes Neandertals ate bears so it's hard to say which is the apex predator. There were other large carnivores around (e.g. species of large cat) but there were also a lot of very large herbivores (woolly rhino, woolly mammoth, bison, various large deer species) to support multiple predator species. From a worldbuilding for a fantasy story perspective, large herbivores can present a substantial danger to characters. Especially ones with huge tusks or other defence mechanisms they evolved to protect themselves from the large carnivores/omnivores.
2. from the perspective of what poses an adequate level of challenge to the MC from a plot point of view. In any fantasy story there's a danger of characters being overpowered and never being sufficiently challenged so there's little substance to the story. So you would need to create creatures/monsters in such a way that they are an equal match to your MC. Start with whatever powers your MC has, and what the MC's weaknesses are, and design creatures that are going to match their strengths and exploit their weaknesses. Maybe the creature can have a weakness that the MC finds out about and this enables your MC to defeat them.