Red Leech

Red Leeches are a species of parasitic invertebrates which inhabited Earth during the Jurassic and Cretaceous period, contemporary to the Siluriancivilization. Like the Silurians themselves, they survived, although seemingly in small numbers, into the Cenozoic era to live alongside mankind.

Biology
According to Vastra, the Red Leech of the Cretaceous was a tiny parasite which infected drinkable water and entered the Silurian system, secreting a deadly poison. The creature observed in Victorian London is presumably either an evolutionary descendant or simply the adult stage of such species: it is a cat-sized being with an appearance that suggests it's either an arthropod or a very advanced worm. It has multiple limbs, including a pair of pedipalp-like structures; a small lamprey-style toothed mouth and large full-black eyes, a red exoskeleton and green blood. Though it can move around on land by crawling, it's shown to be very slow and will rather join with a host and be carried by it. If their host dies, the leech will proceed to crawl in search of a new one.

Mrs. Gillyflower, a delusional Victorian woman who infected herself with the Leech, describes the creature as "a survivor", which has "grown fat on the filth humanity has pumped into the rivers"; implying that parasitism is facultative for this species as they can survive on their own and even thrive in polluted water. The Leeches appears to feed on excess of salt on the host's organism, and produce a form of noxious venom which may cause blindness, paralysis or even death, and typically leaves its victims with red inflamed skin.

After meeting with the creature, which she named "Mr. Sweet", Gillyflower managed to cultivate a symbiotic relationship with it by producing an antidote that gave her resistance to the venom, in exchange for Mr. Sweet's "nectar", the nature of which is never made fully clear. Given their large size which makes them hardly inconspicuous, the fact that the Silurians considered them a major threat and the insane behavior displayed by Mrs. Gillyflower when infected, it's fair to assume that the Leeches might have some sort of mental control over their hosts.

Behind the scenes
The name of this species is a reference to a line in the Sherlock Holmes story The Adventure of the Golden Pince-Nez, in which Watson mentions an earlier case which Holmes dealt with: "the repulsive story of the red leech and the terrible death of Crosby the banker".

Sherlock Holmes's status as a real character in the Doctor Who universe seems to vary according to the different continuities: he is definitely a real person in the New Adventures novels; is implied to be completely fictional in the Missing Adventures novel Evolution; is presumably real in the Big Finish audio stories (as it has featured elements from Holmes canon, including the Diogenes Club with his brother Mycroft); and implied again to be a fictional character in the new series, supposedly inspired by the adventures of Vastra and Jenny Flint. Given the new series's continuity, this could lead to the assumption that "The Crimson Horror" is meant to represent the "repulsive story of the red leech" later mentioned by Conan Doyle. This assumption is highly unlikely, though, as the episode never mentions a banker named Crosby.