Giant maggot

The giant maggots were mutated versions of.

Biology
The giant maggots were similar to regular maggots except for their immense size, sharp fangs and thick skin. They were invulnerable to bullets, armour piercing bullets, and high explosive grenades. Pesticides strengthened them. They would eventually develop into giant flies. (TV: The Green Death)

History
The giant maggots were created by the chemicals dumped by BOSS' Global Chemicals. They killed anyone who came near them, due to their aggressive nature and the deadly chemicals. It was eventually discovered by Professor Cliff Jones that his edible fungus was poisonous to the maggots and it was used to wipe out the entire species. (TV: The Green Death)