House flies are the Nogitsune's animal represenation. They manfested as his form of FoxFire.

 

The Dark kitsune could use flies as messengers, if not embodiments of itself. The flies he could summon gave him mind control. Victims just had to have his fly present in their system, then the nogitsune could influence, direct their actions gradually to his will.

 

Victims of this process include: William Barrow, who had a tumor full of flies within his body, Oliver, Derek Hale, Ethan, Aiden, and Isaac Lahey. The nogitsune attempted to get information out of Ken Yukimura by making him swallow a fly.