I guess if you were to really think about it...if monsters were real then they'd probally have the same issues as humans wouldn't they? I mean most monsters are capable of at least ONE conscious thought right? So, they would more than likely be just as mentally stable & unstable as the rest of us right? As far as physical ailments I guess that would depend on the monster & the affliction then.
Earlier I referenced Rosario + Vampire as to what shit would be like, but I think that crappy show UGLY AMERICANS would fit this hypothetical better.