Time fly

A time fly was a large fly, covered in hair with wings the size of tennis rackets, (PROSE: Slow Decay) while some were completely hairless, had green eyes and wings as well as pincers. (COMIC: A Stitch in Time)

Jack Harkness had a past experience involving time flies and noted that if you'd seen them you wouldn't want to mess with them. (PROSE: Slow Decay)

When the Twelfth Doctor accidentally crashed on top of Ethel's sister, Ethel began summoning time flies to attack both the Doctor and Clara Oswald. They would open up portals to all the Doctor's previous incarnations in an attempt to destroy him. However Gretel, Ethel's other sister, helped the Doctor by getting rid of these flies.

The Doctor, with his sonic screwdriver defeated Ethel by turning the flies on her. (COMIC: A Stitch in Time)

Behind the scenes
Their name is pun on the common phrase ‘time flies’.