The Ergon was a creature created by Omega as one of his agents in the universe of matter to act on his behalf.
Omega created the Ergon with his power of psychosynthesis. He considered it one of his less successful creations. (TV: Arc of Infinity) While on Earth, Ace saw it while working at a fast food restaurant. She mistook it for a person wearing a costume. (PROSE: Anti-Matter with Fries)
It used a matter converter, with which it shot Colin Frazer, Tegan Jovanka and Robin Stuart. While the Fifth Doctor wrestled with the Ergon, Nyssa took its gun and shot it. (TV: Arc of Infinity)
Tegan later described the Ergon as "looking suspiciously like a plucked chicken". (AUDIO: The Waters of Amsterdam)