Proto-Indo-European Roots

Root/Stem: *ekwo-
Meanings:  a horse
Cognates: Greek hippos (horse) - an example of how *kw > p in Greek
Latin equus (horse) 
Common Celtic *ekwos (horse) > 
Gaulish epos, eqos, Goidelic *ehwah, Ogham Irish eqa, Old Irish ech, Irish and Scottish Gaelic each, Welsh & Cornish ebol (a colt), Breton ebeul (a colt)
Common Germanic *ihwaz
Gothic aihwa- (horse), Old English eoh, Old Norse jo'r, Old Saxon ehu-, Old High German eha-
Hieroglyphic Hittite asu, asuwa (horse) - this form made some scientists say that *-k- in *ekwo- was palatal *-k'- that changed into -s- in Anatolian languages; 
Lycian esbedi (cavalry)
Tocharic A yuk (a horse), B yakwe
Sanskrit ava- (horse), Mitanni Aryan asvasanni (a stableman) 
Avestan asva- (a horse), Old Persian asa-, Pamir yas', Ossetic jfs
Thracian esb, esvas (a donkey, a horse), 
Phrygian es' (a donkey) 
Old Baltic *as'u-, probably > 
Lithuanian as'va (a mare), Old Prussian aswinan (mare's milk)
Notes: The noun is masculine, belongs often to IE o-stems. 
It is sometimes hard to identify its derivatives in modern languages because of its variable phonetical structure, but we did everything we could. The problem in some languages is that the word for "horse" had a lot of synonims which replaced the very word. That happened in Old English, Popular Latin and most Romance languages (though Spanish keeps yegua (a mare) , Old Prussian and all Baltic languages and in Slavic branch. 
Why was it replaced? We think it was connected with some sacral reasons, as a horse used to be an important part in Indo-European religion iand mythology. And so, as it was done with the word for "bear" in Germanic, Slavic and Baltic - the same can be with "horse".