The Fire Costume is a retired costume that was released in September 2005 and October 2015 in Account Upgrades. Putting this costume on certain marapets will set it on fire! Fire pets are white while specific body parts show flames.
Fire Gonk has a striking resemblance to the Pokemon Ponyta.
Equip the Flaming Weapons set (axe, shield, fan, and hammer) to a Fire pet then view the weapons.