http://www.serebii.net/xy/33.jpg It appears on all of the starters' pages. I have a few thoughts
One thought is that it's connected to the form of the pokemon. The starters don't have different forms (most likely), so they get a pentagon. However, there might be pokemon like gastrodon where one region has one form and another has a different form. Any pokemon with only 1 form might have a pentagon, while other options might have icons like the pokedex icons: http://www.serebii.net/xy/46.jpg That is, different parts of a pentagon.
Another thought is that it just indicates where it was found, with a gift being a pentagon and anything else being the part of Kalos where it was caught.
Anyone else have any thoughts? Or is it something already in the game that I completely forgot about?
Currently awaiting: Lunar Knights 2, AC:NL, Pikmin 3, XY, WW HD, LR:FFXIII, X, LoZ U, FFVXIII Skarmory would slap the hell outta you