@5'4" Submissive
I didn't intentionally place any horrors that are particularly difficult to find. I check the map and didn't find any monsters located in inaccessible areas.I think the possible areas that may have been overlooked include the mist in the 10 o'clock direction, the mist pit and mist in the 9 o'clock direction, and the cliff in the 6 o'clock direction (if the castle gate is facing the 6 o'clock,the above locations are all at the bottom of the map). There are some monsters placed in the mist（but not far from the land） or on the edge of the cliff that should be attracted during exploration and combat. Maybe I scattered them a bit.
Perhaps you didn't attract all the horrors at once during the battle, and there were missing horrors on the edge. You can try using a decoy（trap seems like scarecrow） to lure the horrors over and check some map edges that were not fully explored.

is there a hidden horror some where on the map? i need to find one more horror to unlock all 4 upgrade slots. ive spent far too much time searching the map for it