Unfortunately there aren't any MLP modules for D&D--but I know some people have run MLP D&D sessions at pony meets!
Hasbro now owns Wizards of the Coast, who put out D&D, so maybe someday there D&D modules for it. (Also there is an official MLP RPG, it's just not D&D.)
Yeah, I do wish the dice were marbled or something.
Let's try to guess what classes the ponies are playing . . .
Pinkie Pie - Barbarian
Rarity - Warlock? She just seems like a Warlock player. (Warlocks get their magic by making a deal with something powerful, like Cthuhlu or something.)
Twilight Sparkle - Wizard, because they are the spellcaster class based on book learnin'.
Fluttershy - Druid
Applejack -Ranger probably
Rainbow Dash - Rogue