And MADNESS makes it all complete

@Fortune  
I think that its a fun theory to play with, but not something that should be taken 100% seriously. And even then each character has had only one serious breakdown so there isnt to much to go on (Putting Your Hoof Down was also an experimental “lets make Fluttershy out of character for the plot” scenario, but I don’t necessarily think it constitutes as a breakdown given the situation). And lets face it, we are pretty much guaranteed 2 more seasons of mlp, so we are likely to get more of those moments (and possibly worse ones for the characters who didn’t have it to bad yet).

@Background Pony  
Just going by what I’ve seen on the show itself.  
True, they weren’t created with psychological issues, but they were still made with flaws and from those flaws, these entertaining moments come about.
 
It’s just funnier to think that Ponyville is just a mental ward posing as a town (I have also never read either of those stories. I’m far more familiar with the parodies than the actual works).

@Fortune  
All interesting headcannons, but still irrelevant. None of that is actually, and its sad people come to these conclusions. Lauren Faust didn’t create these characters with a list of psychological problems. The crazy moments were just cases in which the writers wanted to have fun (Which is especially true for Fluttershy. AKR said herself that she just wanted to write Fluttershy in a completely crazy way because it was interesting).  
My point is, going crazy doesn’t mean one IS crazy. I don’t like to think of any of these characters like that. Not to mention alot of people who support those theories are on board the Cupcakes and Rainbow Factory train.

The sanity spectrum runs from Dash (general anxiety, but mostly collected) to Applejack (prone to incredibly poor decision making) to Rarity (rather melodramatic, but at least it’s a safety valve) on the side of “Sane” to Fluttershy (a whole lot of repression that is terrible to behold when it’s actually loosed) to Pinkie (An odd view of the world wrapped up in some some extreme esteem issues) to Twilight (Where to begin?) on “Insane”.

Rarity’s breakdown wasn’t really that bad. Not anywhere like the others. She just got depressed, which is understandable given the circumstances. And she didn’t go overboard and start acting crazy. Dramatic, but not crazy.
 
Rainbow Dash’s was more of a nervous wreck.
 
Applejack was just tired and you don’t think straight when you’re tired.
 
The other three (especially Twilight) were completely nuts. At least Pinkie had a reason for it.

@Background Pony  
I think that was “Art of the Dress”, when she locks herself in her room following the disastrous first fashion show.

Which episode was the Rarity breakdown in?

My Little Psycho
 
(sorry I couldn’t come up with something clever about nuts)

Every family tree has some nuts in it.

Yes, we’ve all gone some different kind of nuts.

fandom in a nutshell