Sega has produced a lot of Sonic games over the years (with varying levels of success), including a series of racing games that started with Sonic Drift. That game was never really able to out-perform Mario Kart as the de-facto kart racing game, but it did spawn the first mobile Sonic racing game way back in 2005.
It was called Sonic Kart 3DX. Sega released this mobile sonic kart racer on the Sonic Cafe, a short-lived mobile platform that only existed for a very specific line of flip-phones that were only available in Japan. On top of that, you had to pay a subscription fee of ¥315 per month, which was on top of your regular cell phone bill.
Needless to say, Sonic Cafe only lasted a few years. Sega hadn’t really figured out the whole monetization on mobile thing yet, and the hardware hadn’t really evolved to the point where you could play decent games on your phone.
This meant that Sonic Kart 3DX disappeared along with Sonic Cafe in 2007. Since then it’s been lost to time, but recently it’s reappeared online where it can be played through the use of emulation software.
Sonic Kart 3DX wasn't exactly cutting edge for 2005 gaming. Mobile phones at the time weren't powerful miniature computers like they are today, and it really shows in the footage below. There's some extremely based 3D environments, pixelated graphics, and very limited sound effects other than the level music.
Otherwise, the game is pretty typical of a racer. There's a speedometer in the corner, a map, a timer, and two camera angles to choose from. You can race against other Sonics online or do personal time attacks, and that's about it.
Compare Sonic Kart 3DX to the sort of Sonic game that someone can make inside another game as a hobby, and you really see how far we've come in the past 15 years.