I created this term a while back in 3.5 and I was using that in context of mechanics of dnd.
in my humble opinion, monster types should be treated as classes, and cr like levels.
if that's the case, all types should be equally viable from 1 to 20.
when you compare dragons (and outsiders) they are overpowered for their cr and often are the majority of high cr creatures..
it is this...discrepancy...is why I called them "mary sue" monsters because they get this power increase, not because of the cr, but because they are dragons (and outsiders).
This bugs me a great deal.
I should be able to make a cr 20 animal, aberation, monstrous humanoid or hell even fey of a similar level while still following the rules that should scare the bejesus out of the characters with the same feel as a cr 20 dragon or outsider.
Instead we got vast discrepancies between creatures of the same type, let alone "mary sue" monsters (outsiders and dragons) that are hideously more powerful then other creatures of the same cr.
the monster shouldn't be scary because it's a dragon, but because it's a high cr encounter.
I asked a simple question on rpg.net.
Namely how could you make a cr 20 animal that would be an epic threat.
and honestly, we were all grasping at straws.
It's one of the reasons why I created the megafauna template in Dark Revelations - The Role Playing Game.
It now gave an ordinary animal an edge by being naturally resistant to magic.