Here's a link to an article on antique dog collars and padlocks -

In the Middle Ages, spiked collars were worn by dogs that needed to be protected as they guarded livestock. Since dogs were an upper class status symbol, ornate collars were standard… gems and precious metals abound. In the 1500s when the middle class expanded in Europe, dog ownership increased so that working classes owned them. A padlock collar became en vogue at this time since, if lost, the real owner could prove rightful claim if his key opened the dog’s padlock.

Engraved Scottish Antique Brass Dog Collar with Padlock C 1850 "John Russell" | eBay

Features: Fully functioning heart padlock. This item comes with a matching small key which will open your padlock; simply attach it to your dog's collar.

Dog Collar ID Heart Padlock Dog ID TAG - Etsy

ANTIQUE ROVER SMALL CHARM BRASS PADLOCK LOCK WITH KEY - DIARY? DOG COLLAR LOCK? Padlocked Collars could Prove Ownership during Middle Ages
During the Middle Ages, before Rabies tags or identification on collars, owners proved ownership of their pets by placing a large padlock on the dog collar that only they held the key for. These collars were often ornate and a sign of wealth and power.

Dog Collar ID Tag, Heart shape Padlock Dog ID Tag

Fabulous unique vintage Crown Trifari 12K gold filled hammered collar necklace choker with padlock heart shaped charm dangle lock closure. This wonderful dog collar style necklace hails from the late 40s early 50s according to the Crown over the T without copyright Trifari hallmark and would add a unique retro mod look to your jewelry wardrobe! A wonderful statement piece from Trifari! Collar measures 13 inches around and is approximately 1.2 inch wide. Hallmarked on the inside Trifari…

Using PADLOCKS and CHAIN as Dog collars! - Datehookup