If any of the drugs previously mentioned makes you see monsters,i must have taken a light dose.Most psychedlics only warp what is already in your field of vision,but yes sometimes it could be so warped that you might percieve the vision as a monster....or maybe the monsters are in your head. The only thing i can think of through research is DMT...and people talking about seeing DMT elves...thats right,you heard it....DMT elves.Fucked up shit.Sometimes when i trip i feel like something is going to pop out at me all the sudden,but it never does.