There was a Golden Demon winning Duel (from 2007) that was on display in Warhammer World. It comprises a Space Marine Chaplin on a bike fighting an Eldar Exodite on a giant lizard.
The Exodites were a concept that had Eldar using giant lizards as mounts and large mounts for weapons (in a similar way to Elephants or Squiggoths). It was never taken further by Citadel, though some Epic concepts were sculptured and cast, but never went on sale. The concept proved popular with some and as a result there is a range of unofficial material on the web, and some lovely models such as this one.