I'm guessing the Empire was just sick of seeing all these fancy dual-monster kits and the outrageous monstrous cavalry units given to some of their worst enemies. Apparently, though, in the Warhammer universe becoming better and more elite as a unit is not achieved through training or even by acquiring new weapons or a bigger, shinier codpiece. No, it's all about getting a new epic mount.... I guess.
So without further ado...
I guess they look pretty cool. I mean, they are definitely not Mournfang Cavalry because the Empire already has a bunch of cavalry units, so nothing exclusive there. What do you give to an army that has pretty much everything, right? So there yo go, dear Empire players.