My Illogical Theory
I think it might be something like a sneak peek for the next season, or something to signify the new season. Think about it, heaps of Pokemon fans know of all 718 Pokemon. If they saw a new Pokemon they'd probably go berserk because it would obviously mean a new generation/new season of Pokemon!! :D
Or, it might be something to do with Pokemon that were planned, but never happened. I'm not sure about this, but I know there's only one instance in the first episode of Pokemon when Ho-oh appeared. Ho-oh may have been planned to be part of Gen I, but instead got revealed in Gen II, with it's anime footprint left in the first season. Or, it might be planned for the next season, but it might be something like an easter egg. It will remain shrouded in mystery until the next season or next generation of games where they are all 'OOOOH THAT'S WHO IT IS ASJIFOAJSDF', and maybe not exactly be a 'sneak peek' since that would be way to early. Just something to leave you a bit confused and to keep you watching until next season.
Hope I helped. :)