So kewl was he that he managed to trick the goddess Athena into trading eternal life in exchange for a measley half-assed purr (it wasn't even the sincere kind!).
Unfortunately for Fox, Athena's brother Apollo knew better. Quite pissed, Apollo descended down to Earth and batted Fox quite viciously on the nose.
"You weasel!" Apollo shouted, "You took advantage of my little sister! Give her back her eternal life NOW!"
Of course, Fox refused, and being super-kewl, stuck his tongue out at Apollo.
So Apollo, now uber-pissed, punished Fox by taking away all of his kewlness, leaving only his fluffy adorableness behind. His weasel-like kewlness lost, Fox whimpered quite pitifully and ran away like an adorable, sissy little coward.
And that, my friends, is how foxes were created.