Some incredible animals that could serve as models for immortal aliens:Immortal Animals
Lobsters never die of old age, only by violence or disease. They just keep growing. And they get more fertile as they grow older and bigger. Glass sponges can live to 15,000 years. There’s a microscopic organism called a bdelloid that prolongs its life by stealing DNA from other species to repair its own.
The centuries-long lifespans of some turtles and bowhead whales make multi-centenarian humanoids seem less unlikely, whether on distant planets or living secretly among us on Earth.
Also check out a link on the above-referenced page to a list of creatures that can remain active with their heads removed, as well as a snake whose head briefly stays “alive” (and bites) after being severed (sort of like the karaoke demon on BUFFY THE VAMPIRE SLAYER). Zombie animals!
Margaret L. CarterCarter's Crypt