These heads are gacha style items, sold in 'blind bag' format where you can't tell what you're getting till you open it. The theme for this set is '3D decapitated character heads'. 

In the CG mock up pamphlet you see at the right, the idea looks ok in theory. The smooth CG with its good proportions and nice coloring could easily convince anyone that keychains of it could be a good idea. (look at that Eggman though! Like big blue glass orbs shoved in where his eyes should go...yikes)

In practice, however, they're a little strange. Having 3D heads, not just flat "art charms" of the heads gives off a slight "broken action figure" vibe, but add to that the mis-coloring problems and giant noses, & you've got a few more problems.

Yes they're gacha, yes they're cheap, but without Eggmans' collar, he has raw neck stump. (it should be black to help "isolate" the head) Then, Metal Sonic gets a black nose, nobody's ears get color (common for Tomy, but look...they're colored in the photo pamphlet) SS has smeared red from his eye, and Eggman has no teeth. (mouth is just pink) So while these have a better selection than the full-bodies above, they're...a little lacking in design.
Interestingly, these turned up at Sainsbury's (a super market) but that's in England. They can also be found in a machine in Crystal Peaks near Sainsburys. They have also be found in the Netherlands at a store in Almere City called "Beeldverhaal". 

Discovered by: VicLeeW