The Collaborative International Dictionary of English v.0.48:

Ichneumon \Ich*neu"mon\, n. [L., fr. Gr. ?, lit., the tracker;
   so called because it hunts out the eggs of the crocodile, fr.
   ? to track or hunt after, fr. 'i`chnos track, footstep.]
   1. (Zool.) Any carnivorous mammal of the genus Herpestes,
      and family Viverrid[ae]. Numerous species are found in
      Asia and Africa. The Egyptian species (Herpestes
      ichneumon), which ranges to Spain and Palestine, is noted
      for destroying the eggs and young of the crocodile as well
      as various snakes and lizards, and hence was considered
      sacred by the ancient Egyptians. The common species of
      India (Herpestes griseus), known as the mongoose, has
      similar habits and is often domesticated. It is noted for
      killing the cobra.
      [1913 Webster]

   2. (Zool.) Any hymenopterous insect of the family
      Ichneumonid[ae], of which several thousand species are
      known, belonging to numerous genera.
      [1913 Webster]

   Note: The female deposits her eggs upon, or in, the bodies of
         other insects, such as caterpillars, plant lice, etc.
         The larva lives upon the internal tissues of the insect
         in which it is parasitic, and finally kills it. Hence,
         many of the species are beneficial to agriculture by
         destroying noxious insects.
         [1913 Webster]

   Ichneumon fly. See Ichneumon, 2.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mongoose \Mon"goose\, Mongoos \Mon"goos\, n.; pl. Mongooses
   1. (Zool.) A species of ichneumon (Herpestes griseus),
      native of India. Applied also to other allied species, as
      the African banded mongoose (Crossarchus fasciatus).
      [Written also mungoose, mungoos, mungous.]
      [1913 Webster]

   2. [Tamil manegos.] A Madagascan lemur (Lemur mongos).
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

mongoose
    n 1: agile grizzled Old World viverrine; preys on snakes and
         rodents