This is on realms, and I haven't tested this on single player, but when you double tap the jump button while on a horse while wearing an elytra, you end up in the odd state of still being in the riding position on the horse, but you've been rotated 90 degrees forward, as if you activated the elytra normally, and end up inside the horse, but you can still move around normally, but return to normal when you press the jump button again.(see image below, it's hard to explain).