Here is a nice look at these uncommon erasers! These are figural erasers produced by Toy Island, and released around the time of Sonic Adventure 1. However, they were difficult to find, even when new. They were advertised on the box-backs of other items, so you may have seen the photo before. Each eraser is a mini fully sculpted figure and you get Sonic Tails Knuckles & a Chao. The features are cut-in (hard to see in photo) and each one is a solid color. Note that the actual erasers are much less shiny than the ones in the box-back scene. 

 You can tell by the sort of tall head and hunched pose that this is a Toy Island item. Their Knuckles action figures all had this type of look. (Chances are, they cast the eraser after re-sizing a mold). Look how the actual erasers differ from the ones on the box-back photo. Knuckles has his fists up rather than down, Tails is reaching out & Sonic's hands are higher. The only one that's nearly the same is the Chao.