CyberToy

CyberToys (also called Robotrons) are toy robots that have motors, allowing them to move.

CyberToys
Both Arthur and Buster wanted one in the episode "Nerves of Steal." In the episode, everybody owned a CyberToy. This caused Buster and Arthur to want the toy like the rest of the kids, so Buster stole one from Jim's Drug Store. They were eventually prompted to return the toy by feelings of guilt, but were caught putting it back at the store, and Jim called their parents.

It appeared again, although an alternate coloring, as a background prop in "The Long Road Home." It is unknown how Arthur got this one. When wind from Arthur's open window rolled a pen onto it, the toy activated and walked forward, knocking over El Boomerang's box, causing the ball to be lost once again, until Alberto finds the ball at the end of said episode.

Robotrons
Arthur used to have one when he was younger which appeared in "Arthur Sells Out," but then sold it to James. It was broken, and James noticed the break in seconds after the sale. Arthur bought him a new Robotron in the end, instead of the video game he wanted.

Trivia

 * CyberToys are likely inspired by the long-running Transformers franchise.