(This i just a quick and dirty re-save/repack of my old mod, in the new CK. And tested for a solid 30 seconds.)

Gives you a Power that lets you whistle for the last horse you rode on, and it will come running. If it is too far away it will be moved to somewhere behind you and run up to you. 
Powers are used just like Shouts, but without the cool-down (default: Z).