Plankenteenies are Nanodragons shown in The Incomplete Book of Dragons.
Like all Nanodragons, Plankenteenies did not participate in the Dragon Rebellion.
Plankenteenies are dragons that look a bit like a plant root up close. Their species name implies they are of an exceedingly small size.
Because their name sounds like 'Plankton', it might be assumed they function the same in the food chain - occurring in the billions and serve as food for other creatures.
The Plankenteenie appears pictorially in the Nanodragon chapter in this reference book. Not much detail is given about them.