In light of some of the episodes from the second series, especially the finale, I have come to the conclusion that sometimes what is fanon is more likely to manifest than what is canon. Surely it would make more sense for Celestia, Luna and Discord to be the most powerful sentients in Equestria's reality; the elements of harmony having greater potential due to a gambit made by the Princesses.
It would also make sense for their to be a seventh element that unites the other six, one that's intangible. Also surely the world of Equestria would've started out with nine sentient species, others occurring either by Discord's influence or by interbreeding.
Besides it's been rumoured that Lauren Faust wanted Celestia and Luna to be the only winged unicorns, which gives good ground for me to refuse to acknowledge the second series' finale, unless it's later referenced as being 'all just a dream' (or nightmare as the case may be).