Clinking Men are zombies that shamble very slowly toward their prey, and once near enough, they lunge forward and fall flat on their face whilst attempting to inflict contact damage. Their name may hint they rattle a prison cage bar, like some hapless victims found in the background of prison stages in other games.
Zombie Thieves are just palette swaps and behave exactly the same, albeit being a bit more resilient. Both variants drop the Prison Garb, an armor that provides STR +5 and DEF +20, making it a good item to look for the first time these enemies are encountered.
Furthermore, the Zombie Thief has a chance of dropping the valuable Luck Ring (LCK +50). A good place for farming this accessory is in the room just before entering the Machine Tower, where two of them can be found near the entrance and without any other enemies around.