So I was watching the latest YOCS episode (#103) and it got me thinking about my ponysona's back story and the fact that lately I've been drawing her with a full pony body (plus fins) instead of as a hippocampus, so I thought I should come up with an excuse to make this possible in-universe. So, I created a best friend for her: Sea Mist the unicorn. Basically, Sea Mist is a marine biologist with a talent for underwater exploration, thanks to her trusty bubble spell. Whenever Muddy happens across something interesting on the bottom of the ocean her first thought is to share it with Sea Mist, so the two of them often go on adventures together. In return for Muddy's help underwater, Sea Mist travelled to Canterlot in order to study transformation charms until, with some help, she was able to create one that would allow Muddy to walk on land. And that's why Muddy Waters has a pony form.