Source is more logic based, really, but if you want a tangible link, then check the Bulbapedia or Serebii page for any place in X and Y where you can find wild hordes. There are always only 3 different hordes, with one having a 60% of appearing, one with a 35% chance, and one with a 5% chance. Since 60 is the highest of the three percentages, it must be the common type, and likewise, since 5 is the lowest, it must be the rare type. And by process of elimination, that leaves 35 as Uncommon. I couldn't really put that as a source though, since it would be way too long to put as a source.