Not my art. But I can somewhat understand why you'd think it would be fire. But really, there isn't any strong or heavy connection between Cerberus and fire, just that there are a lot of flames and fire typically in the underworld.
Pretty much that - even in other media (Dante's Inferno [game] and Harry Potter [movie]), the Cerberus is not even close to being 'lit on fire', and the imagery used to depict the creature has absolutely nothing related to fire itself. Besides, there already has been another
-using Cerberus in the Crucible archives anyway - Darkness is a good substitute.
