Pokemons like Porygon and Castform have been created by humans. While Porygon resembles more of a machine, Castform takes on more creature like features. The biggest difference is that Castform can have gender differences, while Porygon cannot.
Still, can they be called actual creatures? They did not appear naturally in the pokemon world, in other words, they are not descendants of Mew, the beholder of all the pokemons' genes. So my opinion is that they are not. What's yours?