Meanwhile, on the other side of the bay…
 
Trixie squirms as her leg bones crumble away, while her tailbone quickly develops inside the muscles that formed her legs. The muscular fibers shift and change and cling onto the developing spine, giving it shape and function. Her separated fin-feet fuse into a traslucid tail fluke. Her lungs mutate into gills, and the gill-slits on her neck start filtering the water. Trixie lets out instinctively her last breath of air, which come out as a string of bubbles. She doesn’t feel pain however as all of this happens.  
Rainbow Dash smiled slyly, watching Trixie’s last bits of bipedal human shape vanish away.  
“So, Trixie… you were saying something about going for a walk, uh?”  
“Rainbow Dash… So… this is what happened to you?” asked Trixie, still surprised and looking at her transformed body.  
“And to Twilight, Applejack, Pinkie Pie and Rarity” continued Rainbow, feeling a bit of remorse at those two names.  
“So, is this some kind of pony-up, like you call it? Do I get to have powers?… I always envied you all having the magic light show and powers. Besides, this tail color fits the Great and Powerful Trrrixie!”  
“Uh…” Rainbow Dash thought really fast, this could be useful. “But of course you will have powers! Together we can find them and even share them!”  
Trixie looks at Rainbow, then at her own body. “My magic show can be even more unique! The Grrreat and Powerful Trixie, the MerMagician!”  
Rainbow Dash mood lighted, she smiled honestly, yet she had an idea about their contagious predicament… If Trixie liked being a mermaid… why contain it? It is cool.  
“Trixie, I think you will need assistants for your show. You came here with your…”  
“Assistants and loyal Illusions, Lavender Lace and Fuchsia Blush? Of course!” Trixie said turning around a bit bombastically.  
Rainbow Dash smiled. “Trixie, this is the start of a beautiful friendship… Let’s go recruitin’. You will need them to be up to speed, or in this case… down to depth.”  
“Trixie agrees! Uh… This is temporary like a pony-up, right?” asked Trixie, a small voice on her head telling her that all of this might be wrong.  
“Of course, Trixie. Of course”, answered Dash… “Temporary as in to the day you expire…” thought the rainbow mermaid, her own other voice telling her that this was definitely wrong… but at the same time, might be fun to see some other folk turn into merfolk.