Birthday cakes don't have to be so serious after you turn 20, 30, 40...heck! Even 100! I love a sophisticated layer cake but compared to a frickin' multicolored horse!? It's not even a question which one I'd rather have my candles in. To wish my dear, beautiful friend a "Happy Birth-nay" this weekend, I whipped up this sweet stallion. It's easy—and I'm not just saying that. It's, like, legitimately one of the easiest cakes I've ever created (and I've made some no-brainers). Here's what you…