Just a stab at Bernards question:

"Did 'Poor Old Horse' exist before Souling, Mumming, etc., or was it written along with the 'script'?"

I guess the horse of souling and mumming was taken to sea to become the "Poor Old Horse". Shanties as we know them, such that we do, Stan Hugil so to speakk, seem to be a product of the so called Great Days of Sail, the industrialisation of long distance trade via sailing. I think that was mostly 18 or 19C activity.

I will take advice M'Lord