Found this very curious Sri Lankan version of the "Frog Prince" tale in Surlalune's The Frog Prince and Other Frog Tales From Around The World. And although it features a frog who turns into a prince, it's pretty different from our standard, Grimm version of the tale of the same name.
This frog prince was not enchanted or cursed to have a frog form, but was simply born as a frog to a poor widow. One day the King proclaimed that whoever brought him the Jeweled Golden Cock from the home of the Ogress Rakshasi should be given half the kingdom and an elephant's load of goods. His mother pounded rice for a living, and one day he strung some of the rice on a date tree, and became a handsome Prince. (One wonders why he hadn't already done this? And what correlation the action had to the consequence? The mysteries of the fairy tale world...)
From there the tale continues for a while like a typical hero tale-the Prince travels, is given magical gifts from several Kings to help him in times of trouble (a piece of charcoal that will turn into a fire-fence, a stone that becomes a mountain, and a thorn that creates a thorn fence). He arrives at the Rakshasi's home and is hidden and protected by the Rakshasi's daughter. She hides him in a trunk, tells the Ogress that the human flesh she smells must be from elsewhere, and the next day provides a distraction so the Prince can get away with the Jeweled Golden Cock. (The Rakshasi and her daughter seem similar to Baba Yaga, who also sometimes has daughters who tend to fill the same helper functions while Baba Yaga tries to eat the hero).
The ogresses begin to chase the hero, and he uses his magical objects to throw barriers in their way. The thorn fence and mountain don't seem to be too much trouble for him, but they are both burned in the fire (even the daughter who helped him! Although the narration does say she all of a sudden starts running towards him to eat him. I had sort of been hoping that the ogress' daughter would become beautiful and marry the Prince).
Then the ending is surprising and sudden. The frog returned to the string of rice that transformed him, ate the rice, and resumed his former frog form. "After he became a frog, the clothes that he was wearing, and the horse, and the Jeweled Golden Cock vanished. Out of grief on that account, that frog died at that very place."
Rakshasa image from Wikipedia
Read the full tale on Surlalune
*Also: I've added the tag "unhappy endings" to this and several older posts. So if you want to find world folklore and/or versions of traditional tales with a tragic twist, they're easier to find in my archives now