The Ergon

The Ergon was a reptilian/avian creature created and used by Omega as one of his agents in the universe of matter.

Biography
Omega created the Ergon with his power of psychosynthesis. He considered it one of his less successful creations. (DW: Arc of Infinity) Bizarrely, while on Earth (presumably in Perivale, where she grew up), Ace saw it while working at a fast food restaurant. Ace, at the time, mistook it for a person wearing a costume. (DWM: Anti-Matter with Fries)

It used an matter conversion gun, with which it shot Colin Frazer, Tegan Jovanka and Robin Stuart. While wrestling with the Doctor, Nyssa took its gun and shot it. (DW: Arc of Infinity)