Well we can't know for sure at this point, but here is are speculations:
I think that Stallord was Volvagia before he died, here comes my reasons. After you defeat Volvagia in Ocarina of Time he is turned into bones (and the scull especially looks very similar to the one of Stalord), their shapes are very similar (long body with two arms and a head with two horns) and they can both breath fire (or whatever it is that Stallord is breathing).
So how did Volvagias body get transported to the desert? I don't actually have an answer to that, but hey, it's Zelda we're talking about! Not everything needs to make sense!
See the similarities.
It's NOT a dragon lol. We SPEAK WITH ALL 3 OF THEM, Eldin, Faron and Lanayru, and also Ordona, throughout twilight princess.
It's probably not related to anything..