Pliny the Elder [1st century CE] (Natural History, Book 8, 10; 11, 40): [Book 8, 10] They [elephants] are liable to extreme torture if in drinking they swallow a leech (the common name for which I notice has now begun to be 'blood-sucker' [sanguisugam]); when this attaches itself in the actual breathing passage it causes intolerable pain. [Book 11, 40] Equally remarkable is the thirst for blood that is even felt by leeches in marshy water; for they too penetrate with the whole of their head. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 11, 5.3): The leech [sanguisuga] is a water vermin, so named because it sucks blood [sanguinem sugere]. It lies in wait for creatures when they are drinking, and when it glides into their throat, or attaches itself somewhere, it drinks in their blood. When it is sated by too much blood, it vomits out what it has drunk so that it may once more suck in fresher blood. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.44): A leech [sanguisuga] is a worm that lives in water. It lacks bones and feet and feathers. It attacks men and beasts to drink blood. With whose blood, when it drinks too much, if often brings death to itself. People voluntarily give themselves to these worms to drink the excess blood from the body; but some are less wise, unless they feel an excessive corruption of the blood. For in a healthy man they draw a more noble and well-digested blood. A leech is said to draw blood from the object of its work. The ones that have wrinkles and lines on their backs are said to be better and more harmless; the worst are the black ones. Andelmus the philosopher writes about these things. But of these the Experimentator says, that the leech vomits the blood which it has drunk, that it may drink fresher blood. It pricks itself with nettles and stinging nettles so that it vomits the poison, if it has ingested something from wild animals or toads or water snakes. When it sticks to the flesh, the more it is pulled, the more strongly it sticks until it breaks. When it draws putrid blood, it kills itself by healing another. It has a mouth shaped like a triangle, so it leaves a triangular wound. The Liber Kyrannidarum says that if you burn leeches and mix their ashes with vinegar and use it instead of plucking hairs, they will no longer grow back. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.92): A Water léeche is called Sanguisuga, and is a water worme, & hath that name for he loveth and sucketh bloud, & lieth in a wait upon them that drink, and when he falleth to the jaws, or cleaveth to any place, he sucketh & drinketh bloud, and when he hath dronke overmuch, he casteth it out againe, and sucketh newe bloud yet againe, as Isidore sayth, lib. 12. and is a worme with some part browne colour, distinguished with some deale red strakes, & is soft of body evenlong, and plyaunt, and his mouth is thrée cornered, and in his mouth is a pipe, with the which he sucketh bloud: and he sitteth uppon venimous things, and therefore when he shall bée set to a member because of medicine, first he shal be wrapped in nettles and in Salt, & is thereby compelled to cast out of his body, if he have tasted any venimous thing in warme water. And in hot time he applieth & setteth himselfe soone to a member for to sucke. - [Batman]