Spike and Twilight Sparkle. The unlikely lovers. The two have been together for sometime, and before they officially tie the knot Twilight wants to have children. So on the night her estrus cycle begins she turns Spike into a stallion, so that they may commit to intercourse for the sole purpose of procreation.

Of course, that doesn't mean they can't have a little FUN. Twilight is in heat, after all, and she gets a little rowdy when in heat.