Sparko is a small robot assembled from paper and a marble.
Behind the Scenes
The Sparko "rolling action figure" was released by Best Brains as part of the "Life Survival Kits" available for purchase from the MST3K Information Club. . He was available for purchase from the start of the club in late 1989 through about mid-1993, when he was replaced by a similar "rolling action figure" of Tom Servo.
Sparko never appeared on the show or anywhere other than on his own instruction/assembly sheet.
As indicated by the letters in Sparko's lower right edge, he was drawn by series creator Joel Hodgson. It is possible that Sparko was retired due to Joel's decision to leave the show around the same time.
-  "Life Survival Kits" on Tom's Temple of MST3K Stuff