There are a number of excellent 10mm wolf riders available for Warmaster around the web. The best in my view are from Copplestone Castings.
However, for battles set in Middle-earth, I wanted some Warg Riders. So I purchased some of Splintered Light's 15mm Dire Hyenas and some of Copplestone Castings 10mm Orcs. The Dire Hyenas make excellent Warg proxies and Copplestone's Orcs are the best Middle-earth Orcs I've seen available. Copplestone makes some really nice 10mm miniatures for Middle-earth proxies.
I carefully cut the Orcs from their bases and each other, as some are connected around their weapons & shields. I then simply placed them onto the Dire Hyenas by carefully spreading their legs apart to fit over the Hyena miniature. Be careful not to break off their legs this way.
Once super-glued in place, I had my Warg Riders!
I have base-coated these guy's in Chaos Black and then with a wet-brush of Bestial Brown all over the warg and orc model. Given that orcs seem to have a fondness for all things dirt, the colour combination means that most of the primary painting is achieved this way fairly quickly.
I've had these guys ready waiting to be painted for ages now, so its good to finally catch up on some Warmaster units after completing my Star Trek miniatures that needed painting.