I consider "monster" to be mainly size, but attitude also. For example, my monster fish include:
clown loach (over a foot in length, full grown), black ghost, fire eel, all kinds of plecos, severums, chocolate and blackbelt cichlids, common tiger shovelnose, clown knife, and my fish from the past: tilapia mariae, black doradid, featherfin knife, marble cat, marble goby, ornate polypterus, and my long time lover, my African lungfish,Spike.