Two men were beaten with a dog chain as they walked along Chapel Street, Ripley, on August 26.
They were approached by another male dog walker at 3.15am who ran up to them shouting.
He struck one of them with a metal dog lead on the head and the second man in the stomach.
The man was described as white, in his late 20s, about 5ft 9ins with a shaven head and white tracksuit bottoms.
His dog was a white Staffordshire bull terrier with a brown patch on his ear. Anyone with information is urged to call police on 101.