Congratulations! You beat the game! You defeated the really evil dude, saved the world, got the girl, and can return home a hero. The credits roll and its The End. Or is it...? You load up your game again to find that there is another level available or maybe that cave you saw earlier is no longer blocked off. You head inside and you find IT.
Heads up for possible spoilers.
The Bonus/Secret boss is usually something available after players complete the game or near the end. This guy eats the final boss for breakfast and will probably eat you a couple of times too. Should you manage to beat him you might receive the super weapon of almighty power. Of course by then you won't need it since you already beat the strongest enemy in the game. Of course not all Bonus/Secret bosses are like that but I'm sure you get the general idea.
My personal favorite is the cameo battle from Tales of the Abyss. When I first played it I was caught completely by surprise because I was doing the colosseum battles (on Very Hard difficulty too.) when suddenly I was pitted against a team of protagonists from previous Tales game. These dudes were tough! In addition to there being four of them, they were also able to regularly revive and heal themselves which made me understand what it felt like to be one of the enemies in the game when your opponents would keep coming back to life after you killed them. At one point I had 3 of them dead with just the last one alive when suddenly he used his burst limit which not only revived all his dead allies but also changed the battle music too. The fight ended up taking almost half an hour and used up nearly all my items but I finally won and it felt awesome.
So who/what is your favorite secret/bonus boss. Do you have any funny stories about it?