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.
      [1913 Webster]

   2. A farrier; a veterinary surgeon.
      [1913 Webster]

Easton's 1897 Bible Dictionary:

Horse-leech
   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.