Crocs.
Rainbow Dash was proud to be an athletic girl. Not only did she kick all kinds of rear end in soccer, she also enjoyed rowing, archery, foosball, biathlon, squash, cricket, tetherball, curling, and a whole bunch of other obscure sports almost no one cares about. Mostly, she enjoyed gloating over her fallen foes about how amazing awesome she was, but perhaps somewhere in the back of her mind she felt a little bit of a reward for staying fit and healthy. On occasion, she might have even enjoyed being a team player. It's doubtful though; she still had a lot to learn about friendship, much less teamwork. But the point is, Rainbow Dash exercised a lot.
It comes as no surprise then that Rainbow's feet frequently stank. Besides the obvious build up of salty sweat, she probably had athlete's foot and about half a dozen other types of fungal infections. At least there weren't any oozing infected purulent open wounds. That would have been straight nasty.
Anyway, Rainbow's feet had a dank smell, and while normally she didn't care too much about what other people thought of her (since she already knew in her heart she was awesome), the malodorous stench was starting to scare off potential boyfriends (and possibly girlfriends; pubescent Rainbow was getting mixed signals from her...um... body). Rainbow needed a way to stop the stink. Since actually getting anti-fungal creams from a doctor and changing socks more than once a week was too much work, the lazy student-athlete decided that the simplest solution was probably just to get more air circulation going and diffuse the smell. Fortunately, there existed the perfect shoe for this:
Crocs.
Yes, Rainbow willingly stuck her feet into foam (or are they plastic? We may never know.) pocket-marked clogs. True, the stink subsided a bit, mostly because her feet were dryer. But was it worth the ostracism it gained her? Let's look at the reactions of her closest friends:
"Sweet apple pie in a chipmunk's belly, what in tarnation are y'all wearing?!"
"Look, I'm a party person and I love bright colors and festivity and balloons and cake and streamers but those last three aren't really relevant but anyway I'm all for crazy colorful footwear, but some lines you just don't cross."
"Darling, I'm going to pinch myself and hope I wake up from this fashion nightmare. Those are simply atrocious."
"Sweet Celestia! Did neon changelings crawl onto your feet and die?! I didn't think it was scientifically possible for shoes to be this tacky!"
"Are those my punishment for turning into a demon and trying to take over Equestria with teenage zombies? BRB I'm gonna get myself banished again."
"Um... I know like, you can wear whatever you want, and uh, that's great, but, well, if you want my opinion... please, please, please burn those monstrosities and then bury the ashes in a lead and clay-lined biohazard container in the middle of the desert at least a mile underground. Then build me a time machine so I can go back in time and prevent this from happening."
As you can see from their reactions, Dash's shoes didn't go over too well. Parents, talk to your children today about the dangers of wearing Crocs. Together, we can eradicate this menace to society.