Easton's Bible Dictionary (1897)
Occurs only in Prov. 30:15 (Heb. ‘alukah); the generic name for any blood-sucking annelid. There are various species in the marshes and pools of Palestine. That here referred to, the Hoemopis, is remarkable for the coarseness of its bite, and is therefore not used for medical purposes. They are spoken of in the East with feelings of aversion and horror, because of their propensity to fasten on the tongue and nostrils of horses when they come to
drink out of the pools. The medicinal leech (Hirudo medicinalis), besides other species of leeches, are common in the waters of Syria.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
HORSE'-LEECH (the adherer), a well-known kind of worm very common in all the stagnant waters of Palestine, Prov 30:15. It fastens itself within the nostrils or mouths of animals as they drink, and will suffer itself to be nearly torn in two before relaxing its hold. Its thirst for blood, never satisfied till its body is completely filled, may illustrate the insatiable cravings of lust, avarice, and cruelty.