Here's a good shot of one of Toy Island's individually carded keychains. Each of these keychains features a 3D figure of the character, and were most widely available on one of the numerous re-releases of their SA1 large action figure line. Each figure was paired with it's matching keychain, except for Eggman, who had a chao instead.
The individual card keychain seen here is far less common. It's not known where these were available, but they were not available for all that long, helping them be scarce. With the card here, you can see who else was available as well as they were marketing under "Sonic X" at the time. The figure itself is hard PVC with the metal loop attached at the top.