Red leech
From Tardis Wiki, the free Doctor Who reference
The Repulsive Red Leech was a small animal native to the planet Earth where it lived around 60 million years ago. 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. The Red Leeches survived the catacylsm that forced the Silurians into deep hiberation.
One such creature survived into the Victorian era where it was discovered Mrs Gillyflower who called it Mr Sweet. She survived a symbiotic existence with the Red Leech where she made use of its poison in an attempt to purging the planet of life. (TV: The Crimson Horror)
