Board Thread:Dragon City Questions and Answers/@comment-99.249.175.235-20130515225600/@comment-14284922-20130812233539

It is worth mentioning that a lot of this information is out of date since the Light/War update. It is mentioned on many threads that when trying to breed an exlusive breedable dragon (such as Plankton, Poo, Great White, or Lava) the use of a legend dragon is highly recommended.

(I'm not one to say required, but I've yet to hear of anyone getting an exclusive after Light/War without a legend dragon).

So if you are looking for a Plankton dragon, my advice would be Legendary + Terra.