The Devil Costume is a retired costume that was introduced all the way back in November 2006 along with the Angel Costume. It had its infernal comeback in its September 2020 Account Upgrades set along with the announcement of Poera Prey. Putting this costume on certain marapets will turn it into a devil! Devil pets have spiraling horns, extra-sharp teeth, a pointed tail, fire right behind them, and a pitchfork with occasionally goblin-like clothing.
Equip your Devil pet with Staff of the Devil and then view that pet's weapons page.