There’s a species of jellyfish, Turritopsis nutricula, which is potentially biologically immortal. Even when sexually mature it can revert to it’s polyp stage and then mature back into an adult again, apparently indefinitely. Which is kind of cool.
As far as we know, this is unique in the animal kingdom. It’s interesting to speculate how it might have evolved, especially since most Turritopsis in the wild are eaten in their plankton stage without having reverted to the polyp form. Puzzling.
It is also ridiculously beautiful.