Horse breeders and riders measure horses at the withers, or the shoulder blades, since it's a stable point that doesn't move, unlike the horse's head and neck.
Horsey folks will know this word as denoting the high point on a horse's back, where the shoulder bones meet, and you can use it for that part of a dog's body as well. It's an old word in English, and it maintains a dignified silence about its true origins — and why, like scissors and pants, it's a plural denoting a single thing.