Words for fox in Celtic languages.
| Old Irish (Goídelc) | sinnach / sindach = fox |
|---|---|
| Irish (Gaeilge) | sionnach [ˈʃɪn̪ˠəx / ʃɪˈn̪ˠɑx / ˈʃɪn̪ˠax] = fox |
| Scottish Gaelic (Gàidhlig) | sionnach [ˈʃun̪ˠəx] / seannach [ˈʃan̪ˠəx] = fox, bagpipe reed |
| Manx (Gaelg) | shynnagh = fox |
Etymology: unknown
| Irish (Gaeilge) | madra rua [madəɣ ‘r̪ˠuəɣ] = fox, red fox |
|---|---|
| Scottish Gaelic (Gàidhlig) | madadh-ruadh [madəɣ ‘r̪ˠuəɣ] = fox, red fox |
Etymology: from the Old Irish madrad (dog) and the rúad (red) [source].
| Scottish Gaelic (Gàidhlig) | balgair [bal̪ˠagɛrʲ] = fox |
|---|
Etymology: unknown.
| Proto-Celtic | *loɸernos = fox |
|---|---|
| Gaulish | *louernos = fox |
| Old Irish (Goídelc) | loarn = fox |
| Proto-Brythonic | llowern = fox |
| Old Welsh | louern = fox |
| Middle Welsh (Kymraec) | llewyrn = fox |
| Welsh (Cymraeg) | llewyrn = foxes |
| Old Cornish | louuern = fox |
| Cornish (Kernewek) | lowarn = fox |
| Old Breton | louuern = fox |
| Middle Breton | louarn = fox |
| Breton (Brezhoneg) | louarn [ˈluː.aʁn] / luhern = fox |
Etymology: from Proto-Indo-European *h₂wlop- (fox) [source].
| Welsh (Cymraeg) | llwynog [ˈɬʊɨ̯nɔɡ] = fox, a cunning, craft or willy person, bundle llwynoges = vixen, female fox llwynogyn = little fox |
|---|
Etymology: from llwyn (bush, shrub, brake, thicket, copse, grove, arbour, woods, forest) and the suffix -og, from Latin lignum (firewood, tree), Proto-Indo-European *leǵ-no- (that which is collected) [source].
| Welsh (Cymraeg) | cadno [ˈkadnɔ] = fox; craft person, foxy, deceptive day; bundle of straw used for constructing an air passage in a stack of corn not well-seasoned; skid, scotch, trigger cadnawes = vixen, female fox |
|---|
Etymology: probably from the personal name Cadno, from cad (battle), from the Proto-Celtic *katus (battle), from the Proto-Indo-European *kéh₃tus (fight); and the suffix -no, from the Proto-Celtic *gnāw- (knowing) [source].
Words marked with a * are reconstructions.
Sources: Wiktionary, Am Faclair Beag, MacBain’s Dictionary, In Dúil Bélrai English – Old-Irish Glossary, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau

Hello there. In Welsh, I think the term you were looking for under *loɸernos is: llewyrn.