Giga Bowser (Final Smash)
When preforming this move, Bowser transforms into Giga Bowser. In Brawl and SSB4, Giga Bowser can be freely controlled, moving and attacking normally with a moveset similar to Bowser's.
In Brawl and SSB4, Bowser transforms into Giga Bowser, granting him his immense strength and modified moveset from Super Smash Bros. Melee. While in this form, he is invulnerable to knockback, but can still receive damage, which may leave Bowser severely weakened and open to KOs after the transformation expires. He gains no new abilities as Giga Bowser, but his existing abilities are augmented, with many attacks gaining elemental effects.
In Super Smash Bros. 4, Giga Bowser retains all of his moves from Brawl despite Bowser receiving a revamped moveset; moves such as his dash attack, forward smash and back aerial remain unchanged functionally. The transformation lasts for 660 frames (11 seconds) in Super Smash Bros. Brawl and 480 frames (8 seconds) in Super Smash Bros. 4.
Giga Bowser first appeared in Melee as a hidden boss in Adventure Mode. To get to him, Adventure Mode must be done on Normal difficulty or higher, in 18 minutes or less, and no continues. Giga Bowser also appeared in the final event in Melee as an opponent, alongside with Ganondorf and Mewtwo. Giga Bowser returns for Brawl and SSB4, but this time he is Bowser's Final Smash and not a boss. Since he's now Bowser's Final Smash, he is a semi playable character. Aside from being a semi playable character and appearing in Bowser's Final Smash trophy (for both Brawl and Super Smash Bros. for Wii U), Giga Bowser does not make any other appearances.
Strictly speaking, Giga Bowser did not appear in any Mario games, despite being considered a Mario character. Giga Bowser is likely based on the tradition of final boss fights in Mario games involving Bowser growing to gigantic size; this first appeared in Super Mario World 2: Yoshi's Island (featuring a baby Bowser).
Changes from Melee to Brawl
Changes from Brawl to SSB4
Super Smash Bros. Brawl
Super Smash Bros. 4
Super Smash Bros. 4 adjusts Giga Bowser's attacks by either changing their damage output, or adjusting their knockback values.
Playability through hacking
Other oddities and glitches