Felsteel Annihilator is dropped from the final boss Archimonde in Mythic HFC. It's now a 100% drop wow mount in the current expansion, but will become rare in the coming Legion. For Mount junkers, it is wise to get Felsteel Annihilator before the new expansion.
Felsteel Annihilator is a mini sub-Iron Reaver; its shape is similar to the engineering mount Sky Golem.
In current expansion, most of the guilds started farming Mythic HFC; the accompanying Felsteel Annihilator also became the symbol of the current expansion. Defeating the mythic Archimonde to get a mythic mount, the upcoming end of Ahead of the Curve: The Black Gate, class trinket, and epic, mythic gears, is extremely attractive for a wower.
For those who did not have the Felsteel Annihilator, there are still plenty of time to get it. What are you waiting for? Take action now, as it is now a 100% drop mount, only 1-3% drop rate while in the new expansion.

Felsteel Annihilator：