Monsters are one of the best parts of fantasy. They can make your world more diverse and interesting, and they can be a fantastic terror to expose your characters to! Monsters can be defined as “creatures without an identified culture who exist outside of the established ecosystem”. An example for you: a forest with deers running happily around in it, then a balrog appears. Pretty obvious who doesn’t belong in that scenario.
When creating monsters for your story there are a couple of things to keep in mind to make it successful. The most important thing is: don’t be too abstract. If the reader doesn’t have anything to connect the monster’s appearance to it won’t be intimidating or understandable, which means they’ll be more focused on trying to make sense of what kind of monster is attacking your beloved MC than being scared shitless.
An example of a bad case of monster-writing: the Mortal Instruments’ other dimension things.
An example of a good case of monster-writing: Harry Potter’s various monsters. What J.K. Rowling does nicely is that she mix her monsters features with everyday adjectives that we all understand and know intimately.
Other things you need to keep in mind when writing your monsters are specifics. If they’ve got super strength, how strong are they really? If they got high intelligence, how intelligent are they? Do they have special abilities or powers, what are the limitations of those? And so on. Depending on these abilities or traits you should also keep in mind to make sure they’re balanced. You don’t want a monster that is so over the top powerful that nothing can stand against it. If it can fly, perhaps it has bad night vision? If it’s super fast it might not have great intelligence. Make sure it has weaknesses that would make it vulnerable against other species.
For example, cats are agile but not the strongest (except for the big felines of course), whereas dogs are strong but not the most agile. They have different strengths and weaknesses that balance each other out. You should always apply this way of thinking to your monsters, then you shouldn’t have any issues with overpowered beasts that can eat your main character on one unfortunate afternoon stroll. And that just puts a dampener on the whole story.