Plutonium, an element, is indeed found in nature. Just not in a pure form.
Some of its ores are concentrated enough naturally to be harmful.
The most stable Pu isotope has a T1/2 of 80 million years, so I doubt you’ll find it in nature. It’s not on a daughter path in a decay chain so it’s not being produced by decay, like Po or Ra.
Were you thinking of polonium?