Twilight Sparkle
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NG Magic User
Should she be a Mage, or a Sorcerer? If I strictly follow canon, Twilight is your classic bookish, academic Mage. The trouble is, I don't really LIKE Mages all that much, and my go-to class for D&D is almost always the Sorcerer. From a gameplay perspective, I'd prefer Twilight to be a Sorc; this is especially important if Twilight is to be the Bhaalspawn (she IS the obvious choice for that role, canonically speaking). The main obstacle to making her a Sorc would be her Prime Requisite score... Twilight is an INT based caster, and that requirement is non-negotiable. I'm NOT making a dumb, pretty Twi. Now IIRC, BG treats Sorcs as INT based casters, rather than CHA based casters; if this is the case, an annoying ruleset error would actually work in my favor for once. The other option would be to make Twilight a Wild Mage: I've never used Wild Magic in BG, and it might make for an interesting change of pace. At any rate, I'm going to try and use her dragon familiar (Spike) as often as possible.
Rainbow Dash

CG Kensai
Rainbow is my favorite pony, and if she was playing DnD with us, you just
know she'd roll up a munchkin. So, what better choice for her than my favorite class, the class which also happens to be the most power-gamey one in Baldur's Gate, the Kensai-to-Thief dual? Unfortunately, this means that Rainbow's alignment will be a problem. Rainbow is Chaotic Good, yet Kensai are barred from taking Chaotic alignments (thanks, 2nd Edition) I could just fudge it and ignore the game engine's alignment value (it doesn't have much of an effect on gameplay, anyway), but it's an annoying glitch all the same. Also, since Rainbow is best pony, I'm leaning about 70-30 in favor of making Rainbow the Bhaalspawn, over Twilight. I'm still not sure which scenario would be worse, though: being saddled with a less interesting pony as my mane character, or knowing that I've blatantly ignored the obvious roleplay route for the sake of my baser, metagame instincts.
Applejack

LG Tank
Applejack is a warrior and a tank. As an outdoorsy country girl, she'd probably do well as a Ranger, but I envision her more as a heavily armored knight (specifically, a lowborn hedge knight, if you're an ASoIaF nerd, think Ser Duncan the Tall). I don't want to make her a vanilla fighter, because that would be boring. I'm leaning towards making her a Berserker, since the kit is very strong, and I haven't had much opportunity to try it in-game. However, like Dash the Kensai, this kit will present me with an alignment problem, as the stupid game bars Berserkers from picking Lawful alignments. Another option might be to make her an Inquisitor, which would give her the right alignment, but a Paladin makes it harder to roleplay her farmgirl roots.
Pinkie Pie

CG Bard
Pinkie Pie is a bard. Obviously. The Jester kit is a natural fit for her, and though I like the look of the Skald too, so Jester Pinkie is probably the way to go. Only real problem is that the Bard class seems like a throwaway character. I've got a dim, half-formed idea about turning Pinkie into a dedicated Thief and/or multi-Thief, which would free Rarity up to be a versatile Cleric/Mage support character, but I'll most likely stick with Pinkie the Bard.
Rarity
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LG Caster/Rogue
Rarity is a dual class caster and rogue. The obvious roleplay choice would be to make her a Mage/Thief (a "courtesan/agent" type of thief), although I've already got a fully dedicated arcane caster (Twilight) and a half caster (Pinkie Pie). I'd really like a second healer in my party, so I might make her a Cleric/Thief. As mentioned above, in an ideal world, I'd like to make her a Cleric/Mage, but I need an early-game Thief and I do feel that she'd fit the bill.
Fluttershy

NG Cleric
Fluttershy is a Cleric. I guess she COULD be a druid, but 1) druids tend to be more combat-oriented than Fluttershy would ever be 2) druids tend to be very grim and morally neutral, all nature red in tooth and claw, whereas Fluttershy is a total goodie-goodie 3) druids suck compared to clerics. Ideally, Fluttershy would be a Cleric of a good-aligned nature goddess, with access to the domains of Nature and Healing (or Nature and Good). However, the Baldur's Gate engine does not support clerical domains as far as I know, so I'll probably be stuck taking the Lathander kit.
I'm also toying with some possible alternates, like Shining Armor, the Paladin, Zacora, the Druid, Roid Rage the 'zerker, and Babs Seed the halfling thief. What to do, what to do?