I always had it as "prad", probably from the Penguin Australian Song Book (J.S. Manifold) and possibly also from June Tabor's recorded version. Can't find the book at the moment, but as I recall it explained "prad" as in MtheGM's OP, ie related to paard/pferd.

When I sang it a while back in Preston, Tom Walsh reckoned it was a cant word for horse.

The Routledge Dictionary of Historical Slang by Eric Partridge has prad = horse with references from 1799, also various related items:
prad-cove; a horse-dealer
prad-holder; a bridle
prad-lay; Cutting bags from behind horses (sic!); the stealing of bridles etc
prad-napper, -napping; horse-thief, horse-theiving
There is also a reference to a secondary definition of "prod", but I can't get Google Books to bring that up for me.

Ross