Roller (G1)

Roller (Buggy, in the Japanese show) is part of Optimus Prime_G1, along with his trailer. He is a is a small cart-shaped reconnaisance drone that Prime can use to unobtrusively slip behind enemy lines. He can maintain radio control over it at distances up to 1200 miles. Having Roller present somewhere is like being there himself.

Roller's is apparently "one tough little autobot", and he gets a cool upgrade in Battlestars.

From Teletraan I: The Transformers Wiki, a Wikia wiki.