Answer: Leishmania sp. promastigotes.
The promastigote form is found in the Phlebotomine sandfly host as well as in Leishmania culture, which is in contrast to human infections where the non-motile amastigote form is seen . Of note, I have also occasionally seen rare promastigotes in superficial biopsies of skin ulcers.
Note that the end with the flagellum is ANTERIOR (which is counter to what you would generally expect). You can especially appreciate this while watching the movie of the promastigotes moving (go back and take a look if you missed it).
Thanks again to Idzi Potters for donating this fun case!