Wikipedia's iTunes history [1] says iPod support came in October 2001 (i.e. when the iPod was released), and Rio One support in December 2001, and doesn't really mention other MP3 players.
At any rate, the faithfulness with which the iPod presented the iTunes music library was great.
January 9, 2001: "Apple Introduces iTunes — World’s Best and Easiest To Use Jukebox Software"
"download songs to popular MP3 players from Rio and Creative Labs with plug-and-play simplicity with no extra software or complicated driver installations required."
Wikipedia's iTunes history [1] says iPod support came in October 2001 (i.e. when the iPod was released), and Rio One support in December 2001, and doesn't really mention other MP3 players.
At any rate, the faithfulness with which the iPod presented the iTunes music library was great.
[1] https://en.wikipedia.org/wiki/History_of_iTunes#iTunes_2