Whilst Jervis Tetch was in prison, an unnamed person took on the alias Mad Hatter in his place. Unlike his predecessor, he was obsessed with hats and stealing Batman's Cowl. When the original Mad Hatter returned to the role, the imposter adopted the alias of Hatman instead.
In his first appearance, the Mad Hatter attempted to steal a trophy from the Gotham Yacht Club, but was foiled by Batman while he tried to rob spectators from a high society horseshow. Tetch was subsequently sent to Arkham Asylum (although that was not revealed until Batman #400, 1986).
Later a different, moustached Mad Hatter appeared, claiming to be Jervis Tetch, but he was revealed as an impostor and subsequently disposed of once the real Tetch re-appeared. After the real Jervis Tetch/Mad Hatter had been sent to Arkham following his debut, a very different Mad Hatter appeared, who claimed to be Jervis Tetch. This Mad Hatter first appeared in Detective Comics #230 in April 1956, and, unlike the original, was sane and sported a gaudy mustache. He was primarily a thief, apparently obsessed with completing his private collection of hats from all nations, cultures, and historical periods. He often constructed various weaponry concealed inside his hats like flame-throwers and buzzsaws.
The headgear he wanted most was, of course, Batman's cowl. In numerous attempts, he tried to de-cowl Batman. After many tries, he was successful, after spraying the cowl with a radioactive substance causing Batman to remove it. No sooner did the Mad Hatter put it in his collection than Batman and Robin arrive. They had traced the cowl with their "super sensitive Geiger counter" in the Batplane.
Later on, in Batman #297 (March 1978), "Tetch" claimed to have gone straight, but that turned out to be a lie. In 1981's Detective Comics #510, it was revealed that he was in fact an imposter when the real Jervis Tetch returned. The real Hatter claimed to have killed his imposter, but the fake Mad Hatter appeared one last time in 1987 in Detective Comics #573.