From Halopedia, the Halo wiki
The Fireball Warthog is a variant of the M12 Warthog Force Application Vehicle. Its primary discernible difference from standard Warthogs is the presence of a "fireball"-themed paint job/ decal covering the majority of the vehicle. The Fireball Warthog has been observed in Scout, Light Reconnaissance Vehicle and Gauss Warthog configurations, sporting no weapon, an M41 Vulcan and M68 Gauss cannon respectively.
The Fireball Warthog is a special skin available for the Warthog unit in Halo Wars. In the original game release, it is included as a downloadable code alongside the Honor Guard Wraith with a purchase of the Halo Wars Limited Edition, though was included for all players in the release of Halo Wars: Definitive Edition. The skins for both vehicles can be toggled on/off as a setting in the in-game menus, in which the Fireball skin will replace all player-owned Warthogs, no matter what upgrade stage they are at.
In the campaign, player-owned Warthogs will also appear as Fireball variants, with the exception of John Forge's Warthog.
List of appearances