No one ever said being a new parent was easy, especially if your wife is a changeling. Regardless, there is a little one on the way, and Rhino and Gel will have to learn how to be parents, as well as dealing with the first pony-changeling hybrid.
Differences are everywhere, some react well to them, some not so much. To the young, everything is different as they learn about the world. So what happens when a pony and changeling, two very different beings, meet at a young age?
A normal stallion gets an unexpected surprise in the form of a changeling newborn, of course just because one change is done on purpose doesn't mean more aren't on the way by coincidence.