Giant Fish I guess could be the one closest to being a 'secret boss'. The hidden dungeon was a dungeon, so you expect to have a boss there, the event was an event so Event bosses are also a given. SAO calls them 'Field boss' and plants them outside the labyrinth. The idea i had relates from a game I used to play called Shaiya, in which every one of the maps - the MMO was split up into 4 maps - had a super-boss hidden somewhere away. Had no quest for it, no pre-requisition and no way of finding the dam thing other than scouring the entire map. The only one I ever found was a humongous Dragon on the 3rd map after I got lost, it was up on the top left corner of the map, well away from everything else.