I'm told this is the symbol applied to Pokemon generated via the PokeBank event from Gen 1 games. It is the latest in a series of symbols, starting in Gen 6, to denote what generation the Pokemon was originally created in. For example, Pokemon originating from a copy of X/Y/OR/AS all have the Pentagon that is the symbol of the Kalos region. Pokemon originating from Sun/Moon have the Flower that is the symbol of the Alola region.
