Guys, Kaben has a point, there ARE secret bosses... The giant fish WAS a secret boss, the hidden dungeon DID have a secret boss... And the event was a rumor that turned out true (info was probably leaked by Heathcliff), SAO is just like any other MMO, there are secret bosses, however, we should be sure to ask Yip whenever we wish to bring one in, and don't expect to fight it alone, you won't be able to, simple as that. Secret or no, a boss IS a boss.
To sum up, I believe there should be secret bosses, but we should talk to Yip about them, and we should avoid trying to be the lone wolf who can somehow beat an event boss (looking at you, Kirito) without any aid... (Which, btw, event bosses should be equal to, or stronger than that floor's boss, as they ARE only there at specific times, or under specific conditions)