As someone who raises horses and other animals, I can say with pretty high certainty that most of the horses were not allowed to "retire". Horses are expensive

As someone who raises horses and other animals, I can say with pretty high certainty that most of the horses were not allowed to "retire". Horses are expensive and time-consuming to care for, and with no practical use, most horses would have been sent not to the glue factory but (at that time) to the butcher and their non-meat parts used for fertilizer.