Here is a question that has been rattling around my head for a while. 
Most fantasy roleplays that feature monsters also have ordinary animals. Everything from the typical farm stuff like chickens, cows and pigs to cats, dogs and snakes. Even more dangerous animals like wolves, bears and sharks may appear. But then we take a step into monster-territory, because somewhere between an ordinary wolf and the dire wolf there is an invisible line. 

So I've been thinking. The line is obvious to us players and readers. Animals are the creatures that exist in the real world and monsters are the ones that doesn't. But how about the characters in the game/book/movie? Surely, to them the difference between a wolf and a dire wolf is just size, just like between a bear and a grizzly bear. So how does one define a monster in a world where both animals and monsters are natural?