A heterotrophic protist that is free-living and utilizes flagellum for locomotion would be classifies as ... ?
I think what you are looking for, and correct me if I am wrong, is a certain phylum of protist.
However, there are two that are representative of the short description given and they are Phylum EUGLENOPHYTA and Dinoflagellata.
I will go over each to help clarify
EUGLENOPHYTA are probably the most well known flagellates. They are usually free ...