Why not a phoenix? I mean it only makes sense. Once the phoenix dies it just turns into a pile of ash and then when it's ready to respawn, the ashes turn back into a phoenix and bam! Ready for the next battle! :D The buff can be renamed to "Phoenix Slayer" and when you have its full fire buff it makes sense! Your have the spirit of the phoenix within you, burning everything you touch for a limited amount of time.
Or we replace the dragon with a giant crab that pincers your shit. "OH SHIT THE DRAGON SET ME ON FIRE IF ONLY WE WERE ALL FIGHTING IN A BODY OF WATER"
...oh wait. The water belongs to the crabs. Make way for the crustacean master race.
Probably because "dragon slayer" sounds cooler than "phoenix slayer".
The dragon of TT was already slain to make room for a silly giant spider. Don't allow another dragon to be deleted to make room for a lesser creature.
Or we can make the Dragon look like a Phoenix.
Flaming body, maybe even reptilian feathers, etc.
Think monstrous nightmare from How to Train Your Dragon (Movie)
