ok first off i'll eat your BRAINS, then i'mma start rockin gold teeth and FANGS, cuz that's what a mother freakin monster do. lol, sorry, nicki minaj verse stuck in my head.
you've already got the idea, all of the above or even your passion for fish keeping can be described as monster. This site supports and has information on practically anything you can get ahold of
I think generally size...and I think generally anything that can't live comfortably in 100 gallons or less?
I dunno, my only 'monster' fish so far are two senegal bichirs, and by the definition I just gave they aren't monsters at all
You could say mini-monsters are things like the sen's, I guess fish that live comfortably in smaller tanks but are either so hostile or such opportunistic feeders that its hard to pick tankmates for them.
it's still a baby or sub adult? lol. size is irrelavant in this discussion. a 3 inch exodon can be more monsterish then a 12 inch red belly piranha, of course more people will fear the piranha