Pork pies

[k.hick]

Why are pork pies sometimes called "growlers"

[Hollerith]

It's because traditionally pork pies have been made of such dubious ingredients - chiefly grease - the resulting internal protestations are most always voiced in one language... Tee-Hee!

[Quizmonster]

This word 'growler' for 'pork pie' probably came into use in much the same way as 'dog' did in the sense of 'hot dog' = sausage roll. Students at Yale University began to refer to the wagons selling hot sausages in buns outside their dorms as "dog wagons." There was also a suspicion during the 19th century that sausages probably contained dog-meat, so that may have helped spread the idea.
Growlers didn't start at Yale, of course! More probably in Leeds or wherever, as it is mainly a Yorkshire usage, I understand. The animal most likely to be called a growler, of course, is the dog. Accordingly, I suspect the dog-meat connection probably applied there, too.