I'm not sure I would call that riding. With all the poses I suspect this was some odd-ball cult practice. I'm sure the archaeologist who had to polish that knob on that anthropomorphic figure in that one panel #3 has some good stories. Probably significant that its the smoothest part of that figure.

The old joke about what they call a chicken fucker in Oklahoma comes to mind here except I think those pre-Turks were cavorting with leopards.