From Middle English hert, from Old English heorot ("stag"), from Proto-Germanic *herutaz (compare Dutch hert, German Hirsch, Danish/Norwegian/Swedish hjort), from Pre-Germanic *kerudos, from Proto-Indo-European *ḱóru ("horn").
Compare Welsh carw ("deer"), Latin cervus ("deer"), cervīx, Lithuanian kárvė ("cow"), Russian коро́ва ("cow"), Ancient Greek κόρυδος ("crested lark"), κορυφή, κορύπτω, Avestan 𐬯𐬭𐬏, 𐬯𐬭𐬎𐬎𐬁, Sanskrit शरभ ("mythical antelope"). More at horn.