Denny's restaurants had a Sonic Underground theme promotion during the height of the show. They offered buy-able plush toys with certain meals. Naturally, they had to have some way to advertise all this was going on, so here is a nice big cardboard stand. It's got Sonic right from the show with his 'guitar' and Knuckles, who they pretty much left out of the show.
Why is he in there then? The other characters pretty much didn't have anything to do with anything Sonic, and they weren't going to hire out an all new plush design for the "1-shot" type chars (Manic and Sonia) that no one would recognize unless they watched the show.
The display is rather nice though, with plastic box for displaying the dolls, and 3D elements, such as the logo. The Knuckles (while stock, and nicely shaded) is on his way to mutating with a puffy cheek and flat shoe-bolts. You can actually see these plushes over on USA Plushes if you'd like a closer view.