I've taught a couple of horses to bow and/or lie down, and it's fairly simple, especially if the horse is particularly motivated by food.
However, a word of caution: I'd be very hesitant to teach a horse any trick where an unsuspecting/inexperienced person might accidentally cue the horse to perform the trick, in case either party is injured. Because of this, I try to make my cues complex, and I also refrain from trick training any horse that might not have a forever home with me.
Teaching a horse to bow and lay down is dangerous. Have fun, but please consider the implications and be careful!