During the late 50s or 60s I read a sci fi story about a man who went to a planet that had humanoid creatures with tails that had been forced into slavery by humans without tails.

The protagonist ultimately discovered that these enslaved individuals were the descendants of humans who had traveled to this world in order to colonize it, but the leaky atomic engines had created the tails as a mutation.

Thank you in advance for any information!

Sounds like it could be an old episode of Dr. Who from the 60's... But I am not even close to being certain about this..?

I too am looking for this book.

A space ship crewman from a planet those sun gives out high radiation levels. Which causes the formation of vestigial tails meets and mates with one of the women on the colony planet you describe. She becomes pregnant thus proving her race is human They find the original colony ship which although ancient still works and fly back to civilization forcing the planets owners to accept the facts and give equal rights to the first colonists ( i.e.. With tails)

After all that I am afraid I have no information on this book. I will keep looking and will let you know if I find it. If you find it perhaps you will tell me.

I think I have found this book.

The Lani People
By Jesse F. Bone

You can read a short version on Google book search.