Giga Bowser is an transformation of Bowser that appears so far in the Super Smash Bros. series.
Super Smash Bros. Melee
If the player beats Adventure Mode within 18 minutes or less without no continues on a Normal Difficulty or higher, there would be a bonus cutscene. Bowser's trophy would reappear on Final Destination. Bowser's trophy gets hit by lighting and he is free from his trophy, but now gets mutated to now what we know as Giga Bowser. Now, the player haves to defeat Giga Bowser within 15 minutes.
He reappears, but now playable as Bowser's Final Smash.