I kinda feel bad for oscars many casual fishkeepers seem to just want them for macho reasons. :< I guess that goes for any fish purchase though, folks need to research what they're getting into!
If you want to count oscars as monstes, I would include them with pacus.I think a lot of people may not think of pacu because they are mostly veggie eaters, but they definitely are tank busters!
Maybe we can make an attitude scale(0-10) to go along with size. Attitude plus size equals monster! Anything with a sum over 10 qualifies!
Pacu: Attitude 0 + Length in inches 10 = 10 qualifies!
Juvie Pbass: Attitude 3 + Length 7 = 10
Jag cichlid: Attitude 6 + Length 6 = 12
And so on. I bet we could argue all weekend long on what defines attitude (appetite, territorial, etc) and where each fish fall, and maybe even moster ratings for different levels (15, 20, Even a 25+ mega moster title, lol).