Derives from the middle ages - huntsmen would frequently take a pig with them on lengthy hunting trips to facilitate the relief of sexual tension whilst away from the homestead.
Commonly used in modern day slang as an insult.
Sarah, fetch my Fuck Pig, its going to be cold tonight without you!
Mark, have you finished with the Fuck Pig yet?
Fucking 'ell Frank, forget Fred, where's Francis the Fuck Pig?