For those not familiar:
A frog was hopping along the shore of a river looking for a place to cross. He came upon a scorpion sitting on the shore. “Hello, friend frog,” said the scorpion. “It appears you are looking to cross the river. I too want to cross. Would you mind carrying me?”
The frog was taken aback. “Why, if I let you on my back to cross the river, you’d sting me and I would die. I don’t think I’ll do that.”
The scorpion immediately replied, “There is no logic to your concern. If I sting you and you die, I will surely die as well, since I can’t swim. I wouldn’t need a ride if I could swim.”
The frog thought a moment and then said, “Your logic makes sense. Why would you kill me if it would result in your death? Come along and climb on my back and we’ll cross this river.”
The scorpion climbed on the frog’s back and off they went to cross the river.
About halfway across the river, the scorpion raised its tail and stung the frog. The frog was both astounded and disconsolate. “Why did you sting me? Now I will die and you will surely drown and die also.”
The scorpion replied, “I can’t help it. It’s who I am. It’s in my nature.”
I heard this parable for the first time in Teenage Mutant Ninja Turtles, believe it or not.
It was during Fast Forward, one of the less popular later seasons of the 2003 show. They get blown a hundred years into the future, and have to fight the evil clones that their nemesis-of-the-season keeps throwing at them (it's superhero sci-fi, just go with it).
Leonardo's clone--who, much like Frankenstein's monster, never got a real canon name--was captured and the guys tried to take the chance to show him more mercy than he'd shown them. Leo believes that since the clone is, essentially, mostly him, he is capable of change. He delivers the parable, and then asks, 'Are you a scorpion? Or a turtle?' Then, like an absolute madlad, deliberately gives him a chance to betray them as a test of his ability to grow. He fails... mostly. There's a bit at the end that gives it some ambiguity, leaves the question open-ended.
Honestly, one of my all-time favorite moments in turtles, bar none. It's rare to see such deep questions about nature and nurture asked and explored in a kid's cartoon.