After a weekend of drinking beer, eating all those chilli dogs, poutine, and whatever else came in contact with your lips, shitzilla is what attacks you (usually on sunday evening) when you tremble as you approach the toilet.
"Oh man, Shitzilla's on the prowl, can you hear him growling?"
"*Whew* Shitzilla really took a round out of me that time"
Unloading a massave turd in the toilet
Damn that was Shitzilla