The Scorpion and the Frog
The scorpion asked the frog to carry him across the river, and the frog asked, "How do I know you won't sting me?" The scorpion replied, "Why would I do that? Then we'd both drown." So the frog agreed. Halfway across the river, the scorpion stung the frog. The frog asked him why he did it since now they would both die, and the scorpion replied, "It's my nature."
This animal fable is meant to illustrate that some behavior is simply natural and will happen no matter how an animal or person is treated. It is also a warning to the frog not to associate with those they know to have a troublesome nature. Variations commonly include a farmer, youth, turtle, or fox in place of the frog, and a snake in place of the scorpion.