From Muppet Wiki

Inkspots are small, furry brown creatures who live in Fraggle Rock. They're often seen scurrying through the background, and they occasionally sing backup.

In addition to appearing on Fraggle Rock, the Inkspots were used in The Muppet Christmas Carol, Muppet Treasure Island, The Jim Henson Hour, the "Kokomo" music video from Muppet Beach Party, Muppets Tonight, and Mopatop's Shop.