As BluePhoenix says the hoof hearts were in theory supposed to indicate the hoof on which the magnet was but this was rapidly realised to not always be the case as in initial releases hearts were on the display side, i.e. the side facing "out" in the box and the same side as the cutie mark, whether that foot have the magnet or not. However, with later releases and in some batches of the same ponies they had their hoof hearts moved back to being on the "correct" hoof. I think the main ponies with the misplaced hearts are Butterscotch, Minty, Moondancer, CottonCandy, TaTaT, Winter Snow, Wysteria and Sweetberry.