Fetters, shackles or leg irons are a kind of physical restraint used on the feet or ankles. The term "fetter" shares a root with the word "foot".
In humans, typically only prisoners or bondage fetishists will wear shackles. A shackled animal is typically either a dangerous animal or one prone to escape.
Metaphorically, a fetter may be anything that restricts or restrains in any way, hence the word "unfettered".
Simple fetter types continue to be used like puzzle lock shackles as the typical slave iron or irons riveted shut for prisoners being transported to oversea prison camps.
First built-in lock often were of a simple screw-type but soon developed into the "Darby" type. In Europe these continued to be used into the middle of the 20th century whereas in the US from the late 19th century onwards many new designs were invented and produced before handcuffs and leg irons of the Peerless type became the standard since several decades.
A development of the last decades are high security restraints that incorporate a cylinder lock that is more difficult to pick than the lock on standard type cuffs.