You continued you suck on your changing penis, as it got so long, you could feel it was like a growing ant-hill from an ant-farm you would've had as a kid. Then once your legs were completely covered in fur, your toenails began to harden, and your own feet began to stretch, while you were still sitting on the grass.
You were too distracted by your penis, and how well it tasted, that you didn't even care about what was happening to your feet, as they extended high to become half the size of your penis, and your toes began to fuse, to match those of horses legs. One of the problems with it though, is that once your hooves come into place, you were going to lose balance. Just then, it was too late, as your toes finished fusing together, and became a pair of hard steel horse hooves in front of you.