I've just recently become a Brony myself, and from watching all the episodes of MLP season 1 and 2, I have come to notice the psychological makeup of these four ponies: Twilight Sparkle, Pinkie Pie, Fluttershy and Rarity.
Twilight Sparkle: She seems to have a mild form of Asperger's Syndrome, she never truly sought friendship until Princess Celestia's advice, her socialization skills were learned in Ponyville, though imperfectly as she seems to prioritize organization and self-study over socialization, that's not to say that she does not care for her fellow ponies, it's just that her personality and interests are more Aspergian than anything else, she also tends to over analyze situations and meticulously explain simple facts. She also panics when things don't go as planned.
Pinkie Pie: She seems to have some form of ADHD and Neurosis, she's hyperactive and her endorphines are way off the charts, always cheerful and happy and she gets excited over the simplest things.
Fluttershy: Textbook case of Social Anxiety Disorder, while Twilight Sparkle exhibits more Asperger traits with her literal thinking, organizational commitment and esoteric interests, Fluttershy exhibits more of a Social Anxiety Disorder personality, with her extreme shyness towards other ponies and her fixation on little animals to vent off her feelings of inadequacy and insecurity from other ponies.
Rarity: Narcissism, She is a good-hearted pony deep inside but she tends to place too much value on physical beauty, she sees herself as very beautiful and has fantasies on joining the Pony Elite of Equestria, most of her lines on the show tends to emphasize aesthetic beauty over everything else.