Fireball is a Boss Attack Item used by Ganon in Hyrule Warriors as part of the Boss Pack DLC Challenge Mode, Ganon's Fury. It is obtained from defeated King Dodongos and is based on the charged Fireball attack used by King Dodongo in Hyrule Warriors. It allows Ganon to fire Fireballs from his right hand that are capable of putting Argorok in a downed state (exposing its weak point).
Ganon (and Dark Ganon) also use it as one of his boss attacks when he is fought as a Giant Boss. Like King Dodongo, he is vulnerable to Bombs/Super Bombs when preparing his Fireball attack. In Hyrule Warriors Legends, it only appears as Ganon's (and Dark Ganon's) boss attack due to the lack of Challenge Mode in the 3DS revision.