Chaos Fox is a transformation undergone by Fox the Brave. It is one of his most used transformations beyond Fire Fox, the other being Feral Fox. In this form, Fox combines with Tai the Charizard to form a huge, incredibly powerful Mega Charizard Y; Tai provides the form and brute strength, Fox provides the voice and personality for the form.
Chaos Fox has the appearance of a Mega Charizard Y, and the form is about as big as Tai, who is large in himself.
Since Fox the the one mainly controlling the form, his personality is the one shown the most. However, Tai's calmer presence lessens Fox's arrogance and stubbornness a bit.
Shadows of Madness: While struggling to save another dimension, after losing as Fusion Fox, Fox undergoes this transformation for the first time. With this, he easily defeats that dimension's maddened villain.
The War of Time/Storm's War: In both stories, Chaos Fox on appears to battle Enerjak.
Revenge of Enerjak: When Fox fights Dark Enerjak out of rage and determination to avenge Braveheart having his Core stolen, Fox first fights as Feral Fox, then somehow transitions into Chaos Fox and easily defeats Enerjak.
Chaos Fox's powers are largely those of Tai. He can use all of Charizard's learnable moves, though Flamethrower, Dragon Claw, and Dragon Tail are his most used moves.
Fox uses Hyper Fox as a segway into Chaos Fox, since that's how he first underwent the transformation.
Chaos Fox is Fox's second "Fusion Transformation", the first being the aptly named Fusion Fox.