Attilan Rough Riders
The Attilan Rough Riders are a mounted cavalry Imperial Guard unit originating from the planet of Attila. They are fierce warriors, born in the saddle (and more than likely dying in it). While they can be deployed en masse, they are more often than not assigned in small units to other regiments that can make use of their swift flanking abilities. In battle they employ foul tempered horses built like tanks (and sometimes incorporating parts from them) and use explosive-tipped lances to fuck up your shit.
They are the vanilla cavalry (even called Rough Riders) for Imperial Guard forces. Their current line are incredibly shitty-looking figures sold for ridiculous prices. Convert your own from some Warhammer Fantasy stuff, fool.