Yes, it's time for another Friendship is Magic Thread.
Twilight Sparkle: INFJ. Twilight's rather a tough one. She's clearly Ni-dominant, but seems to follow it with Ti and Fe. She's a smart pony, quite kind, but stubborn about her opinions and with a tendency to dispense advice unasked. She's prone to enter an obsessive state (Ni-Ti) in which she refuses to accept external information and simply tries to reason things out based solely on her Ni-vision. When pressed hard, she will try to force things and may become physically aggressive.
Rainbow Dash: ESTP. Rainbow Dash, by comparison, is a pretty easy one. She's more or less a textbook ESTP -- brash, with a love of physical challenges and a dislike of rules.
Pinkie Pie: ESFP. Pinkie Pie, a party loving hedonist, is almost a stereotypical ESFP. Some people have been typing her as ENFP, but Pinkie is largely uninterested in ideas, and has trouble grasping how other ponies see the world. She's also very physical and present in the "now," and her first reaction to something new is usually to pick it up and play with it. All these things show Se-dominance rather than Ne-dominance.
Fluttershy: ISFP. Fluttershy's Fi-dominance is pretty obvious, but her N/S orientation much less so, due to her strong introversion and reliance on her Fi. I have her as ISFP largely due to her connection with animals, the general "well-maintained clutter" look of her cottage, and her lack of anything resembling Ne.
Applejack: ISTJ. Serious, responsible Applejack is a pretty clear STJ. She's pretty balanced between E and I, but ultimately her Si is in charge, and she will frequently choose doing what she "ought" to do over what is practical.
Rarity: ESFJ. I would think Rarity should be obvious, but I've seen people tag her as ENFJ. Her shoulds and oughts tend to focus on physical things such as fixing a dress (or nest) properly and not on questions of right or wrong.