The Collaborative International Dictionary of English v.0.48:
Horse-leech \Horse"-leech`\, n.
1. (Zool.) A large blood-sucking leech (H[ae]mopsis vorax),
of Europe and Northern Africa. It attacks the lips and
mouths of horses.
2. A farrier; a veterinary surgeon.
Easton's 1897 Bible Dictionary:
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.