Catchpole was used to name a sheriff's deputy, or a constable, or other officer whose duty was to arrest persons. The word is not now in use as an official designation.
Catchpole is derived from a law enforcement implement found in medieval England. It consisted of an eight foot wooden pole with some sort of noose or barbed fork on one end. Law enforcement officers (usually the Sheriff) would place the noose around the neck of the criminal and use it to lead them around and so forth. Catchpoles are still used today, mostly by animal control officials to ensnare uncontrolled wild animals.