Minecraft wiki entry for Horses, associated text: Use a piston to hold back the horse. Connect redstone such that the piston releases the horse at the same time as a pulse starts down the repeater chain. At the end of a certain amount of blocks (~45 or so), add pressure plates, which are usually more consistent than tripwire. When these pressure plates are activated, use a long string of redstone to lock every repeater in the chain. This will "freeze" the chain and the pulse that started at the beginning will be frozen in place. You can mark the repeater where the pulse is frozen with a block.