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Don't let the misprint in the manual fool you. I don't know why it says they are (and therefore should be weak against Monk types), since I can't find any in-game evidence to that effect.
Here's what they are: https://en.wikipedia.org/wiki/Styracosaurus
It would make sense if all Monsters were weak against Monk types, but again, that doesn't seem to be the case.
Also, Slimes are weak against Guardsmen, but not Monks, so I guess Guardsmen are not Monk types (the manual doesn't list them as being, but I would've assumed that was a misprint if I didn't put it to the test).