Put the summary of the bug you're having here

jumped on horse (skeletal horse first, then normal horse afterwards) and could not get the horse to move.

tried 3 different horses (all with saddles) in the same chunk and none of them could be convinced to move.

I was able to move the horses with a lead, although I could not make them change their elevation, although this may not be definite. I didn't test this a great deal.

The horses moved normally, otherwise.