Green’s Dictionary of Slang

a bargee; thus western pugs, the barge or bargee who navigates down the Thames to London.

1591		Lyly Man in the Moone IV ii: Yes, in a western barge, when, with a good wind and lusty pugs, one may go ten miles in two days.
1603		Dekker Wonderfull Yeare 84: Euen the Westerne Pugs receiuing money there [in plague time], haue tyed it in a bag at the end of their barge, and trailed it through the Thames.
1611		Middleton & Dekker Roaring Girle IV ii: Pish, your western pug!