Fantasia/WMG

The Firebird isn't evil
It's just a nature spirit responsible for destruction. Note how he just burned the trees and not killed the elk.

The Firebird will appear in Kingdom Hearts
Wishful thinking yes, but Chernabog was a random boss fight and a lot of people enjoyed it. Though if the bird was a character in the game, I doubt a boss fight would really fit the mood, more of a race against time ordeal.

The Devil from Fantasia is the Bigger Bad of the entire Disney multiverse and commands all the other villains
It is he who whispers in the ears of villains like Jafar, Maleficent and the others. And they don't even know he's doing it. He's that subtle.