Primal Reversion may be reserved for the pantheon of Legendary Pokemon:
Mew, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina (Maybe), Regigigas, Arceus, Reshiram, Zekrom, Kyurem (Maybe), Xerneas, Yvetal, and Zygarde.
I call them a pantheon because they're basically the "big"...