Flartus guessed it. It's a tool to measure for Ben's collar.
Just like humans, horses can be the same weight and height, but different sizes. Boomer and Ben are both 18.2 hands and 2,000+ pounds, but Boomer's collar is too small for Ben.
The fit of a driving horse's collar is just as important as the fit of a riding horse's saddle. The collar distributes the load of the vehicle around the horse's neck and shoulders (loosely stolen from Wikipedia).