Sonic Jr. was an incomplete Sonic the Hedgehog title for the SEGA Pico, an educational console. The game involved a younger version of Sonic the Hedgehog.
The game was announced at the same time as the release of Ecco Jr., and the game would have likely been similar in concept. An early release catalog interview for Pico entails claims that a new Sonic game was a desired title for the console. The game never made it off the drawing board. In the end, Sega instead released Tails and the Music Maker and Sonic the Hedgehog's Gameworld.
Whether the eponymous Sonic Jr. was a new character, or simply a younger version of Sonic, is unknown. So little of the game exists that all the internet has to go on is one vague article.