This model was the first actual dinosaur added to the army way back in 2008 and is still my favorite model in my collection.
He was made using the Lizardmen Carnosaur (quite obviously!), some bits from the Command Squad kit (legs, shoulder pads), a cut up Terminator torso (saddle), and a combat shield and WFB Slaaneshi Lord halberd, as well as some assorted Space Marine bits and bobs.
Here are some old WIP shots to give you an impression of the conversion work:
Fluffwise these guys are among the most elite and definitely the most fearsome warriors the Storm Dragons have. The Dragon Riders form a psychic mind bond with their steeds to control them in battle. Of course, should that bond be broken for whatever reason, the consequences can be less than pretty.