I had it happen once in space -- I found a planet that had a tribal version of the race I was playing. I dropped a monolith on them.
(New challenge -- wander the galaxy and find your other 11 'tribes', ala Battlestar Galactica.)
I have seen one of my other races show up as an Epic once, though. That threw me for a minute, I was so surprised. I didn't realize until that moment that the Epics are just random larger versions of the creatures already in your local catalog.