I don't know if this has been brought up before, but I couldn't find anything here about it so here goes.
"A horse and a man above, below.
One has a plan, but both must go.
Mile after mile, above, beneath,
One has a smile and one has teeth.
Though the man above might say hello,
Expect no love from the beast below."
"In bed, above, or deep asleep,
what greater love lies further deep.
This dream must end,
this world must know,
we all depend on the beast below."
So our theory is that there is a future doctor traveling back through the series. This is all assuming that the "future doctor" theory that was discovered in Flesh and Stone is true with the different jackets and such. The Future Doctor is the one below. He has the plan and the one that everything depends on. He knows he has to do something and he does it, with no "smiling" or "saying hello". He doesn't want to be seen. He is "below" because he is underneath the story, going through fixing everything behind the scenes. Everything is depending on him. He is traveling "mile after mile" through time and space, "above and beneath". This Doctor is a man, all alone and without a companion.
The Doctor Above (horse) is the Doctor that is in the main timeline. He is in the action and isn't aware of what is yet to come. He is the one with the smile and the one that might say hello. Normally a horse travels with a companion riding it, like how the Doctor is guiding Amy through all of their adventures.
This leads us further to believe something is up with Leadworth. It just seems off. The bit about the dream ending might be a reference to the town, if it indeed isn't real, or to something else that we'll learn in a later episode. Possibly the one with the Dreamlord? I don't know.
We could be crazy and off our rockers but this feels so legit.