...and adding new code into the game isnt currently possible.
It is possible. It's just a lot harder than normal modding. However, in this case, the OP is trolling.
It would be completely impractical to add horses to the game. It would be EXTREMELY difficult to simply add a new sword to the game, and he says he did that and added horses. Heck, it would be difficult even for the MCPE devs to add horses, let alone some unknown self-proclaimed modder working on one of the hardest games to mod on one of the hardest platforms to mod. So, this thread should be locked. Do not even wait for "proof," as there is nothing to prove.
Actually horses could be implemented into the game... with the removal of another mob. for say, a cow! You can resize and retexture any mob to your liking. So you could resize and retexture a cow and you have a horse ;however, you would not be able to ride the horse...