I've been on an MLP: FIM binge lately, and one thing that I've always loved about the show is how relatable the characters are. I've also realized that the show represents mental conditions very well. Conditions like Asperger's (which I have), even though they can give us some challenges, can also be a blessing in a number of different ways. Here are some connections with characters in MLP that I came up with:

Twilight: Asperger's Syndrome (And Sci-Twi probably has it worse than her)
Rarity and Spike: OCD
Applejack: Bipolarism
Pinkie Pie: ADHD
Rainbow Dash: ADD (She and Pinkie both have a lot of energy, but Pinkie has it worse. Rainbow isn't constantly hyper like her and shows that she can sit still for longer, so she would just be ADD, not ADHD)
Fluttershy: Anxiety
Trixie: Narcisistic Personality Disorder
Starlight Glimmer and Sunset Shimmer: Anger management-related disorders
Cranky Doodle Donkey: Depression
Cozy Glow: ODD/Childhood delinquency (and a very severe case of it at that. Queen Chrysalis and Tirek were already awful people, which explains how they became such powerful villians, but keep in mind that Cozy Glow is just a kid! It's kind of scary how she became such a big villian if you think about it)
Princess Luna: PTSD (formerly)

Also remember that some characters might have the same disability, like how Princess Celestia might also have Asperger's and Discord might also have authority troubles. Let me know if I missed any and all that.