So, this is all I've figured out so far, and it's something I could've done in Photoshop, but it's as far as I've gotten. Imagine the horse is a fully rigged silhouette that gallops along until it gets down to the water. Then it steps carefully in before accelerating offscreen, stage left.
The art is a discarded piece I made a year or two ago, but I had to extend it quite a bit to fit the screen size, spending more time painting than learning how to animate. Bad planning. Again. More soon, unless I toss the computer through the window.