Actually I thought a thief could make a lot of sens, especially since that class is known for being more agile with a focus on dexterity over strength and constitution.
Also argue that Rainbow Dash being a Thor parody workout as well and made a lot of sense, with Pinkie Pie being the speedster because she's so hyperactive.
Also, don't forget despite the blue pegasus being the fastest flyer in Equestria... her pink friend was able to more than keep up with her and always catch up.