Carl's Jr. is a restaurant only seen in parts of the USA. Despite not being universal, they still picked up a Sonic promotion for their kids' meals. (though the items are seldom seen, and may be considered somewhat rare) Their promo was in 1996 and included 4 viewers, each with Sonic, Tails, Robotnik or Knuckles. When you look in, you can see Robotnik, Tails, and Sonic stock arts (much like an old viewmaster toy) Photographed & owned by ST. This promotion MAY HAVE been shared by Wendy's. It is likely the promotions were simultaneous if it was. Added info by Sonictoast. 

There is a rare and interesting Robotnick viewer in this set. It's yellow with black accents instead of beige like Knuckles' one. The sticker on the front is rather interesting because it contains an unusual/seldom seen stock of him. 

The robotnik viewer contain 6 slides, each starring Dr Robotnik. While many things are quite reliant on stock art alone, this seems to take an extra step, adding strange backgrounds like space, stairs and a city-scape of some kind. Others are clearly mash-ups like running Robo & Sonic. The non-sensical/out of character Robotnik with a club appears, but with that terrible 'bad eyed' stock Tails, implying he's been hit. Photographed & owned by Sonictoast

The Tails version of the Carls Jr Slide viewer is srather rare. It has a 'leaning Tails' sticker for the front, & is all blue & orange. You can see the Robotnik one's bag (Sonic branded, with instructions) in the center. The slide (top right) has the same Tails graphic as the sticker, with Sonic in the background.