Jellyfish usually die after propagating but the hydrozoan medusa Turritopsis reverts to a sexually immature stage after reaching adulthood and is capable of rejuvenating itself. It does this through the cell development process of transdifferentiation.
The 4-5mm diameter creature is the only known animal that is capable of reverting to its juvenile polyp state.
Theoretically, this cycle can repeat indefinitely, rendering it potentially immortal.
And it has it's own Facebook page.