There was something similar that my old group did. In a game we did where we decided to play with the monster classes, I played a flesh golem while my wife played a dromite from the psionics book. I ended up with my golem being her character's mount, basically I carried her around in a modified backpack and did things like threw her through windows or over walls to do some roguish stuff. Anyways, what we did was counted my character as the one in charge of movement as I was a sentient being. Since I did not contest her trying to ride on my back, no skill checks or even skill ranks were required to ride. In your instance of a wizard polymorphing, I would say the same would apply. Provided that the wizard allowed the character to ride on his/her back, then I wouldn't personally call for any rolls to be made. The only exception would be during combat which I would require rolls to be made in order to stay on (since more than likely "animal" and rider would choose different things to do during combat), but I would allow a bonus of say 1/2 the mount's level to the rider's roll. Or maybe add the rider's charisma modifier as a bonus to the roll.
