'Mongoose' definitions:

Definition of 'mongoose'

noun

Agile grizzled Old World viverrine; preys on snakes and rodents

From: GCIDE

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.]

From: GCIDE

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]