Words for horse, stallion, mare, foal and related things in Celtic languages.

Note: the commonly-used words for horse in each Celtic language are: capall in Irish, each in Scottish Gaelic, cabbyl in Manx, ceffyl in Welsh, margh in Cornish, and marc’h in Breton.

Words marked with a * are reconstructions.

Proto-Celtic	*kaballos, *kapallos, *kappilos = horse
Gaulish	*caballos = horse
Old Irish (Goídelc)	capall [ˈkapal͈] = horse
Middle Irish (Gaoidhealg)	capall, capail = horse
Irish (Gaeilge)	capall ˈkapˠəl̪ˠ] = horse, mare capallach = equine capaillín = pony capall maide = wooden, vaulting horse, hobby-horse
Scottish Gaelic (Gàidhlig)	capall [kahbəl̪ˠ] = mare, colt, horse, small horse capall-aibhne = hippopotamus capall-coille = capercaillie capall-mara = seahorse capallach [kahbəl̪ˠəx] = pertaining to or abounding in mares/colts capallan [kahbəl̪ˠan] = small horse, pony
Manx (Gaelg)	cabbyl = horse, mount cabbyl awin = hippopotamus cabbyl assylagh = mule
Proto-Brythonic	*kėfɨl = horse
Middle Welsh (Kymraec)	keffyl, ceffyl = horse keffylyn = little horse, nag, pony cavall, cauall = horse, steed
Welsh (Cymraeg)	ceffyl [ˈkɛfɨ̞l / ˈkɛfɪl] = horse, nag, hobby ceffyl yr afon = hippopotamus ceffylaf, ceffylu = to put on horseback, put one to ride the high horse, extol ceffylaidd = pertaining to horses, equine, horsy ceffylan = little horse, nag ceffyles = mare ceffylyn = little horse, nag, pony cafall = horse, steed
Middle Cornish (Cernewec)	cevil, kevil = horse
Old Breton (Brethonoc)	cefel = horse

Etymology: uncertain – related to the Late Latin caballus (horse, nag) and Ancient Greek καβάλλης (kabállēs – nag) and maybe Persian کول (kaval – second class horse of mixed blood). Possibly ultimately from PIE *kebʰ- (worn-out horse, nag). Words from the same roots include cheval (horse) in French, cavalier in English and caballo (horse) in Spanish [source].

The Breton word kefeleg (woodcock) comes from the same Proto-Brythonic root, as does kevelek (woodcock) in Cornish and cyffylog (woodcock) in Welsh [source].

Proto-Celtic	*markos = horse
Galatian	*μάρκαν (márkan) = horse
Gaulish	*markos = horse
Old Irish (Goídelc)	marc [mark] = horse
Middle Irish (Gaoidhealg)	marc = horse
Irish (Gaeilge)	marc [mˠaɾˠk] = horse (literary / archaic) marcach = horseman, rider, jockey; cavalryman, Cavalier marcachas = horsemanship marchaigh = to ride marcaíocht = riding, horsemanship, ride drive lift
Scottish Gaelic (Gàidhlig)	marc [marxk] = charger (warhorse – literary) marc-shluagh = horsemen, riders, cavalry marchach = equestrian, mounted; riding
Manx (Gaelg)	mark = horse mark-sleih = horseman markiagh = to ride, riding, cavalier, equestrian, horseman, jockey, rider markiaghey = riding markiaght = drive, equitation, horsemanship, (horse) riding, lift, rider
Proto-Brythonic	*marx = horse
Middle Welsh (Kymraec)	march = horse
Welsh (Cymraeg)	march [marχ] = horse, stallion, war-horse, steed marchaidd = pertaining to a horse, horsy, horselike, equine marchallu = horsepower marchasyn = jackass, male donkey marchdy = stable marchfeddyg = horse doctor, farrier marchfilwr = dragoon, cavalryman, cavalier, trooper marchog = horseman, rider, jockey, mounted warrior, knight
Old Cornish	march = horse
Middle Cornish (Cernewec)	march = horse
Cornish (Kernewek)	margh [ˈmaɾx] = horse marghek = knight, rider margh-leska = rocking horse marghnerth = horsepower marghogeth = to ride (a horse) marghti = stable
Old Breton (Brethonoc)	marh = horse
Middle Breton (Brezonec)	march = horse marcheg, marhec = horseman, rider, knight marecat = to ride (a horse) marheguez = to ride (a horse), to dominate
Breton (Brezhoneg)	marc’h [marx] = horse, easel marc’h-tan [marxˈtãː.n] = motorbike marc’heg [ˈmar.ɣɛk] = horseman, rider, knight marc’hegkaat [mar.ɣe.ˈkɑːt] = to ride (a horse) marc’hegañ = marc’hegezh [marˈɣeːɡɛs] = to ride (a horse), to dominate marc’hegiezh = chivalry, cavalry

Etymology: thought to be from the Proto-Indo-European *márkos, which is also the root of the English words mare and marshal, the French word maréchal (marshal), and related words in other languages [source].

Proto-Celtic	*ekʷos [ˈe.kʷos] = horse
Celtiberian	ekua- = horse
Gaulish	epos = horse
Primitive Irish	*ᚓᚊᚐᚄ (*eqas) [exʷah] = horse
Old Irish (Goídelc)	ech [ex] = horse
Middle Irish (Gaoidhealg)	ech = horse airech = packhorse
Irish (Gaeilge)	each [ax] = horse, steed (archaic) eachach = abounding in horses eachaí = horseman, jockey, equine eachaire = horse-attendant, groom each-chumhacht = horse-power eachmharcach = horseman
Scottish Gaelic (Gàidhlig)	each [ɛx] = horse each-aibhne = hippopotamus each-coimhlinge = racehorse eachach [ɛxəx] = pertaining to or abounding in horses, horsy eachaire [ɛxɪrʲə] = equerry eachan [ɛxan] = small horse, yarnwindle eachlach [ɛxl̪ˠəx] = horse groom, jockey
Manx (Gaelg)	agh [ax] = steed, riding horse aghee = equine aghlagh, aghragh = equestrian eagh = horse, racehorse, riding horse, steed eagh marrey = sea horse eagh-veg = hobbyhorse
Early Brittonic	*epālos = foal
Proto-Brythonic	*eb [ɛːb] = horse *ebọl [ɛˈbɔːl] = foal
Old Welsh	eb = horse
Middle Welsh (Kymraec)	ep, ebawl = colt, foal ebawluarch, ebolfarch, ebawlfarch = colt, young horse ebolyauc, eboliauc = in foal, capable of bearing a foal
Welsh (Cymraeg)	ebol [ˈɛbɔl / ˈeːbɔl] = colt, foal, sucker eboles [ɛˈbɔlɛs] = filly, foal ebolaidd = coltish, frisky, playful, wanton ebolfarch = colt, young horse cyfeb = mare in foal ebolig = coltish eboliog = in foal, capable of bearing a foal
Middle Cornish (Cernewec)	ebel = foal, colt
Cornish (Kernewek)	ebel = horse
Old Breton (Brethonoc)	ebol = horse
Middle Breton (Brezonec)	ebeul = foal, filly
Breton (Brezhoneg)	ebeul [ˈe.bøl] = foal ebeulan, ebeuliañ = to foal ebeulez = filly keneb = mare in foal

Etymology: from the Proto-Indo-European *h₁éḱwos, which is also the root of the Latin word for horse, equus, and the English word equine, and related words in English and other languages [source]. The horse goddess, Epona, may be related as well.

Proto-Celtic	*uɸorēdos = horse
Gaulish	*werēdos = horse
Proto-Brythonic	*gworuɨð = horse
Middle Welsh (Kymraec)	goruytaur, goruit, gorwyd , gorŵydd = steed, horse gorwyddfarch = (war-)horse, steed
Welsh (Cymraeg)	gorwydd = steed, horse gorwyddfarch = (war-)horse, steed

Etymology: from the Proto-Celtic *uɸo- (under) and *rēdo- (to ride; riding, chariot), from Proto-Indo-European *(H)reydʰ- (to ride). Words from the same Celtic roots include palfrey (a small horse with a smooth, ambling gait), Pferd (horse) in German, and vereda (path, lane) in Spanish [source].

Proto-Celtic	*(φ?)lārek- = mare
Old Irish (Goídelc)	láir = mare
Middle Irish (Gaoidhealg)	láir, lair = mare
Irish (Gaeilge)	láír [l̪ˠɑːɾʲ] = mare An Láír Bhán = the Milky Way láír bhán = hobby-horse láíreog = little mare, young mare, filly, well-built girl, woman
Scottish Gaelic (Gàidhlig)	làir [l̪ˠaːrʲ] = mare
Manx (Gaelg)	laair = mare laaireen = small mare

Etymology: possibly from PIE *pōlH- (animal young), which is also the root of pony and foal in English, pollo (chicken) in Spanish, and poule (hen) in French [source].

Proto-Celtic	*kanxstikā = mare
Middle Welsh (Kymraec)	cassec, kassec = mare
Welsh (Cymraeg)	caseg [ˈkasɛg] = mare
Old Cornish	casec, cassec, casac = mare
Middle Cornish (Cernewec)	casek = mare
Cornish (Kernewek)	kasek = mare
Middle Breton (Brezonec)	casec, casecq = mare
Breton (Brezhoneg)	kazeg [ˈkɑː.zek] = mare

Etymology: from Proto-Indo-European *ḱonḱ- (horse) [source]. Words from the same root possibly include henchman in English, hengst (stallion) in Dutch, and häst (horse, knight) in Swedish [source].

Proto-Celtic	*stirrākos = small animal, chick
Old Irish (Goídelc)	serrach = colt, faol
Middle Irish (Gaoidhealg)	serrach = colt, faol
Irish (Gaeilge)	searrach = colt, faol searrachúil = foal-like, lively, flighty
Scottish Gaelic (Gàidhlig)	searrach [ʃɛr̪ˠəx] = colt, faol, filly searrachan [ʃɛr̪ˠəxan] = little foal searrach-ruadh = buzzard
Manx (Gaelg)	sharragh = faol sharraghoil = faol-like

Etymology: from PIE *stirp- (progeny). Words from the same root possibly include estirpe (lingeage) in Spanish, and sterpo (dry twig or branch, brushwood) in Italian [source].

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic