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.