Imagine this conversation in a possible TV episode:
: Isn't it something in your program which can make you turn yourself into a horse and carry me out of here?
: Wrong hologram. Besides that, I'm a doctor, not a donkey.
: But I'll die here if you can't help me!
: Hmmm. Ok then!
The Doctor then turns himself into a centaur and set off through the wilderness with the dude on his back and in the background we hear this song:
"I've been through the desert on a horse with no name
It felt good to be out of the rain
In the desert you can remember your name
'Cause there ain't no one for to give you no pain