Spiteflayer is a level 55 aggressive rare carrion bird that can be found just south of Nethergarde Keep in the Blasted lands. (I have indicated where he can be found in yellow on the map below.) He generally drops either an Imperfect Draenethyst Fragment or a Flawless Draenethyst Sphere, each of these items begins a quest. When turned in you will receive a "junk" bag for the imperfect one and a chest for the flawless. The junk bag usually contains a green item and the chest will have a blue.
He is also especially desirable as a hunter pet as he has a totally unique model. He is the only tamable two-headed yellow carrion bird in the game.
Spiteflayer can be found with NPCScan. You will need to add it, his NPCScan ID is 8299.
You can also use this macro to help find and tame this guy:
/cast Tame Beast
Before the Cataclysm his spawn timer was somewhere between 10-16 hours. I think it has since been reduced but I don't know for sure.