These are the models @Xinus22 (on Twitter, among other places) created. They
were meant to be higher-quality versions of the Sonic R models, similar to
the ones Jon Burton showcased in https://youtu.be/RfW89U6rWzg

I was going to help them port these models to Sonic R, but for various reasons
(mostly just me being burnt out on Sonic R stuff), it never went anywhere.
That said, they should be ready for importing, once someone writes a proper
model importer. (Veninator1207 on GameBanana seems to got it figured out, so
maybe ask them?)

I'd assume Xinus would be perfectly happy for you to use these models, with
credit to them, but check with them first beforehand.