Pokos are the currency used on Olimar's home planet, Hocotate. This was first introduced Pikmin 2 and the plot concerned them. Olimar and Louie had to pay a debt of ~ 
x10,100 by visiting the Planet of the Pikmin, where collecting Treasures can pay off this supposedly immeasurably large sum of money. In comparison, x100 is more than a year's salary for the average Hocotatian. Pokos are ovular, and thin, and are made of some sort of golden colored material.
It seems that Pokos are used in Koppai (the home planet of the new captains in Pikmin 3) as coins that greatly resemble Pokos are seen in the 'Collect Treasure' meter on top of the screen of Mission Mode.