Rizzo the Rat is performed by Steve Whitmire.
Rizzo the Rat was created for Steve Whitmire after he took a fasination to a bunch of old rat puppets made from bottles that had appeared in the background of a skit.
Now Rizzo is one of the forefront characters, with a staring role in Muppet Christas Carol, Muppet Treasure Island, and Muppets From Space.