Since XY, it has been our objective to breed/raise every Pokémon to have the pentagon. With ORAS, we have extended this to SR-ing legends, so we also have them with the pentagon.
This leaves us with mostly the event legends plus a few friends. Here is a summary of what we have and not have so far.
1. Gen 6 event legends released and we currently have (within the group): Celebi is from Bank.
Jirachi is from last year's shiny event, which was already distributed.
Deoxys is from ORAS
Shaymin, Keldeo, Victini are from the Pokescraps event. I believe Bunsel has them.
Darkrai is from the UK GAME event, which I have and will be distributed.
Arceus is from the JPN movie event, which Bouncer has.
Diancie from last year's and the most recent event.
Hoopa from the JPN movie event, which I also have.
Thanks to bouncer, we now have a Gen 6 Manaphy in the group.
2. Event legends not yet released in Gen 6:Apart from Volcanion (yet to be officially released), Mew, Meloetta, and Genesect are the ones that have yet to sport a Pentagon.
3. Other Pokémon that cannot be bredThese are not legends but still belong to the Undiscovered & Ditto egg groups.
Competitively speaking, distributing an Unown (for the sake of it) isn't worth it but an Imposter Ditto can be cool. But how would we manage one? DexNav? Safari chain? or IV-edited Powersaves?