Roller was the secondary unit of the original Optimus Prime, contained in his trailer when he was in vehicle mode. It resembled a small six-wheeled moon-buggy-like vehicle. Roller was generally used for reconnaissance.
Roller only appeared twice in the US animated series. It was never used in the Marvel comic, though it was mentioned as existing in his Transformers Universe comic-book entries.
One wonders what happened to Roller following the death of its primary unit- presumably, it died along with Prime.