I agree with Harbinger, and many others. A Monster is just as much an animal as animals are, it's intent and it's desire to act upon it's monstrous side makes it a monster. You could say a rabid dog would be a monster due to it's feral nature. But let's talk Elephants the first people who saw these animals outside of their native habitat likely thought them monsters. Since few creatures look the way an Elephant does. They're massive animals with a thick trunk and two long tusks. Anyone who's never set eyes upon them as they first charged the battlefield likely would have thought them demons, with a arm that could crush a man and spears for teeth. Now we know better but the fact remains anyone laying eyes upon a creature so odd and so massive would easily confuse it with a monster just upon looks alone.

Now there's also another detail no one's talked about. How easily can a normal man kill a monster. In many Games, Fantasy and Myth it's Heroes, Adventurers or Knights. Men or Women who are highly trained and equipped to slay monsters. And that's what makes a monster dangerous, your average Farmer cannot hope to slay a Cockatrice, a Manticore or other such creature. While they can more or less kill Wolves with some difficulty and maybe Bears if needed albeit with more danger. Monsters kill average Men, it's why Geralt of the Witchers is called upon, because he can slay beasts no other can. Why it's always been Heroes to kill the Dragon. The average man cannot hope to deal with such creatures, without specialized weapons and equipment. But most animals can be slain by the hands of a single man, be it with guns or spears.