O.E. deor "animal, beast,"
from P.Gmc. *deuzam, the general Germanic word for "animal" (as opposed to man), but often restricted to "wild animal" (cf. O.Fris. diar, Du. dier, O.N. dyr, O.H.G. tior, Ger. Tier "animal," Goth. dius "wild animal," also cf. reindeer),
from PIE *dheusom "creature that breathes,"
from *dheu- (1) "cloud, breath" (cf. Lith. dusti "gasp," dvesti "gasp, perish;" O.C.S. dychati "breathe;" cf. L. animal from anima "breath"),
from base *dheu-.
Sense specialization to a specific animal began in O.E. (usual O.E. for what we now call a deer was heorot; see hart), common by 15c., now complete. Probably via hunting, deer being the favorite animal of the chase (cf. Skt. mrga- "wild animal," used especially for "deer"). Deer-lick is first attested 1778, in an American context.