The present invention is a locking device that can be integrated onto a leash, and which provides a solution to the problem of slip-on collars over tightening or being inconvenient to use. The locking device comprises of a main body, a lock piece, a hook, and one or more arms. The one or more arms are connected within the main body to engage with the hook and the lock piece to lock a width of a collar portion which is formed from the pieces integrated onto the leash. The main body is configured to slide on the leash, and the lock piece is fixed onto the leash and when the lock piece slides into the main body, the main body is prevented from sliding. The hook is connected to a first end of the leash and when inserted into the main body forms the collar portion.