The problem is april fools isn't really a theme lo. You play pranks on april fools, people don't usually dress up. I still think you could pull off the switched rider/horse without spending too much... Go to the fabric store to the scraps area (where they sell everything super cheap) and find yourself about a yards length of material, then craft a tie you could put around your horses neck. Go to the used clothing store and find some kind of cute hat, cut ear holes and attack some string to tether it (use a quick release knot) to the top of the headstall and slip his ears through so it doesn't accidently come off. And I'm sure yo could find a horse mask somewhere, or you could make one out of paper mache. Just use a cardboard base to get the right shape.