Who knows? Maybe just an imaginary creature with some fins on its back that happens to look like a dinosaur. The problem is that armchair archaeologists think "this looks like that" has significance when it usually doesn't.
Hang on. Is he showing this is proof that ancient civilizations had shrinking rays and could turn tiny dinosaurs into rocks, and build them into arches?
We honestly have no way of knowing (unless this is some sort of common religious iconography, which it might be) what whoever carved it was thinking and why at this point. But I think 'dinosaur' is probably the worst answer.
Uh, I've actually seen it. It's located in Cambodia in the Ta Prom temple (also called Tomb Raider Temple, because they filmed parts of the movie there). It's a beautiful temple just a couple of km East of Angkor Wat.
I've heard that archeologists believe it depicts a water buffalo with some leaves in the background.
Until today water buffalos are being held as work animals to carry stuff, pull weagons and plow fields.