According to Otakukart, the unique feature basically refers to the achievements players get from battling a good number of gyms. It could also mean catching a particular kind of Pokemon — be it rare or not. Or perhaps, it’s synonymous to acquiring a specific requirement or something.
With the addition of Gen 2 Pokemon GO monsters, Medals can certainly be a thing to look forward to. The publication also states that the arrival of two new Pokemon types — namely, Dark and Steel — could signal the arrival of the aforesaid feature. After all, both Umbreon and Steelix are expected to be unleashed anytime soon. So, in a sense, it’s only right for the studio to create a specific set of medals for these special species.
Apart from the aforementioned monsters, Legendary Pokemon could also be included. Heck, these types of creatures are even apt for having medal rewards. After all, they’re considered on top of the line. It’s just a matter of time, though, before anything of these can happen.
It holds true that Niantic has yet to give an official word about the feature’s arrival to Pokemon GO. However, it’s also true that lately, the developer has been all ears on every concern/suggestion the fandom has. And take note: Medals are one of the things fans have been dying to see in the game.