After Princess Twilight made it possible to pass through the magic mirror at any time, the Doctor decided to run some experiments on it from the TARDIS. After finding out that the mirror served as a permanent link between two universes that could stay in perfect harmony so long as their counterparts didn’t cross over too often or didn’t interfere with normal happenings, then exploration could be safe. With these assumptions in mind he decided to send the TARDIS through to see what would happen with the relative dimensions. Once through the interior began to jumble and quake, altering to the laws of the human dimension. The Doctor also had lost his equine form in favor of the homo sapian form.
 
 
Once on the other side the portal was having difficulty materializing the exterior in it’s relative size, but eventually it did succeed, making the TARDIS land with it’s usual thump sound. The Doctor will be out shortly once he’s gotten acquainted with his new body.