For example, Pinkie is known to be bipolar (goes from Manic to Depressive in a shot) and is certified she's pretty unstable. Applejack is a workaholic and overly dependable pony that can work herself to exhaustion. Rainbow Dash could be the most normal, but she's also an obsessive fangirl (first the Wonderbolts, then Daring Do) that can get herself overly carried on on her fanaticism (an incoming danger to her friends is the only thing known to snap her out of it). As I remember, those are certified reasons worthy of spending a lot on therapies to get over (Dashie's the lightest of the bunch, but she may still need an intervention). Spike's probably the most normal of the bunch, honestly. He has to have a lot of patience to deal with a town where EVERYTHING can happen.
Yeah, I guess. If you ask me, Rainbow Dash, being the show creator's favorite, was given a less complicate personality. Just an athlete with an over inflated ego and a hard case of fangirlism. The rest, have an appointment with my shrink, which I'm sure moved to Equestria after leaving me with my Dysgraphia, ADHD and whatever else she found on me. And I mean it, I haven't seen her in 15 years and I'm sure she left her desk right after I stopped going because I visited the place and her plate's gone.