"Mr Sweet" was the wholly fictitious name that Winifred Gillyflower gave to a red leech she discovered in the year 1893.
He resided on Gillyflower's chest, giving her his "nectar". He was immediately recognised by Madame Vastra as being from the late Cretaceous Period, and a known menace to the Silurians. When Strax killed Gillyflower and Sweet tried to escape, Ada Gillyflower killed him with her cane. (TV: The Crimson Horror, PROSE: The Crimson Horror)