while i do believe humans once existed in Equestria, i don't buy that. i mean think of evolution as a tree with primates as one branch and equine as another. those two once shared a common place but grew apart, and like branches won't ever fuse again without some sort of intervention.
my personal headcanon is that ponies/dragons/griffins/changelings were made to inhabit the earth by the earth after man wiped himself out, this time she made her dominant inhabitants far more in tune with herself: I:e giving them weather control, magic, sentient animals. this was done to keep them from following our path, however they eventually found relics of man and began to rebuild his creations, giving them leaps and bounds. this explaining why helicopters exist but no cars or planes