My baby Jag is very nasty to all of my MUCH larger fish, such as my red belly pacu that is about 10 times bigger than him. So I would say managuense aka (jaguar cichlid)
i now have 2 mostly white oscars.not so mean.i loveto watch them beg for food..
but i used to have this fish, i know its not really a monster but i had a betta (crackers was his name) that would thrash and threaten, puff up and hit the side of the tank open mouthed at anyone who came into my kitchen.. a little tiny opac( spelling?) he was all white.. tried to give him some large guppies for friends once, they were 2-3 in long and he was only 1.5 in long and he chased them ragged.. so i had to take them out. so Crackers had to live alone... he is still the meanest fish ive ever had.