neck and crop
Uncertain, but crop may refer to the backside of a horse, so that a horse that fell neck and crop had both its neck and backside hit the ground.
"neck-and-crop." YourDictionary, n.d. Web. 26 May 2018. <http://www.yourdictionary.com/neck-and-crop>.
neck-and-crop. (n.d.). Retrieved May 26th, 2018, from http://www.yourdictionary.com/neck-and-crop