The idea that something has to be super big or look like some off the wall alien to be monster is completely asinine, in my opinion.
That being said: Tiger Barbs, Leopard Ctenopoma, a FW Puffer, or maybe a Black Tail shark... not the most colorful fish, but those are just afew I like. There are lots of possibilities out there.
Here is a "mini-monsters" thread. http://www.monsterfishkeepers.com/forums/showthread.php?t=253800&highlight=mini+monster
Good luck.
That being said: Tiger Barbs, Leopard Ctenopoma, a FW Puffer, or maybe a Black Tail shark... not the most colorful fish, but those are just afew I like. There are lots of possibilities out there.
Here is a "mini-monsters" thread. http://www.monsterfishkeepers.com/forums/showthread.php?t=253800&highlight=mini+monster
Good luck.

Maybe a look at the definition of "monster" could help refine your ability to categorize.