A horse collar is a part of a horse harness

Horse harness

A horse harness is a type of horse tack that allows a horse or other equine to pull various horse-drawn vehicles such as a carriage, wagon or sleigh. Harnesses may also be used to hitch animals to other loads such as a plow or canal boat....

 device used to distribute load around a horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

's neck and shoulders when pulling a wagon or plow. The collar often supports and pads a pair of curved metal or wood pieces, called hames, to which the traces

Trace (tack)

In transport, a trace is one of two, or more, straps, ropes or chains by which a carriage or wagon, or the like, is drawn by a harness horse or other draught animal. The once popular idiom: "kick over the traces" comes from a frisky animal kicking one or both feet outside a trace...

 of the harness

Horse harness

A horse harness is a type of horse tack that allows a horse or other equine to pull various horse-drawn vehicles such as a carriage, wagon or sleigh. Harnesses may also be used to hitch animals to other loads such as a plow or canal boat....

 are attached. The collar allows a horse to use its full strength when pulling, essentially allowing the horse to push forward with its hindquarters into the collar rather than to pull with its shoulders as it would be required to do if wearing a yoke

Yoke

A yoke is a wooden beam, normally used between a pair of oxen or other animals to enable them to pull together on a load when working in pairs, as oxen usually do; some yokes are fitted to individual animals. There are several types of yoke, used in different cultures, and for different types of oxen...

 or a breastcollar.