A neck manacle was a device used by the Hunters to humiliate the Tosk if they were ever captured alive and taken back to their homeworld in the Gamma Quadrant. It had a small rope attached to it, which the Hunter would use to pull the Tosk behind him.
In 2369, the First Hunter placed a neck manacle on Tosk after he was released into his custody by Constable Odo on Deep Space 9. After the Hunter was incapacitated by Miles O'Brien on DS9's promenade, Tosk's neck manacle was taken off by using O'Brien's phaser. (DS9: "Captive Pursuit")