One jellyfish can age backwards, turning itself back into a polyp.
Turritopsis dohrnii, the so-called immortal jellyfish, can respond to injury or starvation by reverting from its adult form to a juvenile polyp. The polyp then grows into new jellyfish all over again.
Why is this true?
The trick is transdifferentiation: specialised cells change into other cell types. In theory the cycle can repeat indefinitely, although in the wild most of these jellyfish are still eaten or fall ill.
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More about animals
Animals are the easiest place to find facts that sound invented. Three hearts, blue blood, cube-shaped droppings, a punch fast enough to boil water for an instant — every one of these is ordinary biology to the animal doing it. The collection here leans on well-documented behaviour and anatomy rather than folklore, so each entry comes with a short explanation of why the trick actually works and a source you can read next.