Immortal Rats

Naked mole rats are rather ugly, but they age well. More specifically, they are the only mammal that doesn't appear to die of old age: "these rodents live to around 35 years, as opposed to a "regular" rat's six years, and the naked mole rat doesn't seem to actually age before it dies." Specifically:

the chance of dying for the mole rats did not increase as they aged. All other mammals that have been studied have been found to conform to what is known as Gompertz's mortality law, which states that the risk of death for a typical mammal grows exponentially after they reach sexual maturity—for humans, that means the odds of dying double every eight years after reaching age 30.

Sure, they die. But, they don't undergo the physical decay with age that characterizes other mammals whose bodies start to fall apart at a certain point. Female naked mole rats remain fertile pretty much until they die.

Needless to say, naked mole rats will now be at the center of efforts to figure out the aging process so that humans can cheat it as much as possible.

Naked mole rates are also almost cold blooded, which may not be unrelated. Crocodiles and some sharks, both of which are also cold blooded, seem to continue to grow indefinitely, with a case in point being the Greenland shark which is believed to live to be as much as 512 years old, older than any other known vertebrate.