The dragon in the orb in Blackreach. I wonder if it is possible to ride him down there. Any thoughts?

I'm fairly certain the only flying he does is scripted and if you actually do get on him, he won't take off.
But I have no proof, so... you're welcome to try. Haha.

Some dude managed to blow the flesh of a serpent dragon so that it became a skeletal dragon, and then ride it, which shouldn't be possible. We won't know unless someone tries.

PS: The skeletal dragon riding thing? Vid: http://www.youtube.com/watch?v=gKnzQ3R0JiU