YIAZMAT (FINAL FANTASY 12)
Final Fantasy 12, like all games in the series, has plenty of secret super-bosses to hunt down, and the most ridiculously overpowered one (maybe even across the entire series) is Yiazmat. Part-dragon, part-unicorn, this thing is an absolute nightmare to deal with. It has a whopping 50 million HP, which means killing it is guaranteed to take a while. On top of that, when its health falls below 50%, the game puts a significant cap on your damage output, prolonging the fight even further. Yiazmat can also cast spells that completely heal it, and deal devastating damage with several attacks. All of which is to say, it can (and does) literally take hours to beat this guy- more so in the original than in The Zodiac Age, though.
