Vulgar Boatman's posting 15 Jan 04 comes closest to explaining origins.
The forces version is a parody (like most forces songs) of an old broadside piece called 'The Silly Mare and Foal'. It seems to have been quite scarce as I only have one early copy, probably printed in York, as it is in with a batch of broadsides almost all with York imprints in the BL. There are however oral versions as my old mate, Ken Canfield, used to sing it in the 60s. The copy I have I'd guess at about 1820. I'll post it but I'll have to type it out on Word first. Lost too many postings by trying to post direct.