I've painted up three of my Scout Bikers recently so I can use them in my games:
I took inspiration from the following picture of a velociraptor for the Cold Ones:
The conversions were pretty straightforward, I had to cut off one of the Scout legs and reposition it slightly to make them fit on the saddles, but nothing major.
Fluffwise these guys are trainee dragon riders, already veterans from the Tactical companies. They train their psychic skills of mastering animals on raptors, who can be more easily subdued if the mind bond is broken than a monstrous Carnosaur!