Some kind of Demon.
Yeah, I like that idea.
Suppose we get a Burning Legion themed expansion, we get a renegade race of Demon called the... err, "Demon People" (For the sake of giving them a name) that were residents of the Twisting Nether. Something happened with the Burning Legion, they came across the Demon People and Demon People were tricked into following the Legion because the Legion are sneaky like that.
Some of them break away and head to Azeroth to warn us of the oncoming threat.
They're like big and purple and black and green and stuff with tails.
No hooves though.
But horns, nice big, pointy horns.
And fire. Bits of them are on fire.
Well, that's where my imagination ends. Thanks for coming!
(PS, I know little about the Legion or Nether or anything so I have no idea if that's possible)