The Collaborative International Dictionary of English v.0.48:

Hag \Hag\ (h[a^]g), n. [OE. hagge, hegge, witch, hag, AS.
   h[ae]gtesse; akin to OHG. hagazussa, G. hexe, D. heks, Dan.
   hex, Sw. h[aum]xa. The first part of the word is prob. the
   same as E. haw, hedge, and the orig. meaning was perh., wood
   woman, wild woman. [root]12.]
   1. A witch, sorceress, or enchantress; also, a wizard. [Obs.]
      "[Silenus] that old hag." --Golding.
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   2. An ugly old woman. --Dryden.
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   3. A fury; a she-monster. --Crashaw.
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   4. (Zool.) An eel-like marine marsipobranch (Myxine
      glutinosa), allied to the lamprey. It has a suctorial
      mouth, with labial appendages, and a single pair of gill
      openings. It is the type of the order Hyperotreta.
      Called also hagfish, borer, slime eel, sucker, and
      sleepmarken.
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   5. (Zool.) The hagdon or shearwater.
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   6. An appearance of light and fire on a horse's mane or a
      man's hair. --Blount.
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   Hag moth (Zool.), a moth (Phobetron pithecium), the larva
      of which has curious side appendages, and feeds on fruit
      trees.

   Hag's tooth (Naut.), an ugly irregularity in the pattern of
      matting or pointing.
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WordNet (r) 3.0 (2006):

Myxine glutinosa
    n 1: typical hagfish