Red leech

The red leech — metaphorically known to the Eleventh Doctor as the "crimson horror" — 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. Madame Vastra called the species "repulsive", claiming that it was the Silurians' "most virulent enemy". This intense hatred derived from the fact that these creatures secreted 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)