Red leech

The Repulsive Red Leech was a small animal native to the planet Earth where it lived around 60 million years BC. Its existence dated back to the time when the Silurians dominated the world where the reptilian species depised the red leeches. These creatures were capable of secreting a microscopic toxin that killed their hosts.

One such creature survived into the Victorian era where it was discovered by Winifred Gillyflower who named it Mister Sweet. She survived a symbiotic existence with the Red Leech where she made use of its poison in an attempt to purge the planet of life. (TV: The Crimson Horror)