Middle Welsh, used from roughly the twelfth through fourteenth centuries CE, is the literary stage of the Welsh language in which the Mabinogion, a collection of medieval Welsh prose tales drawing on older oral and mythological Brythonic Celtic tradition, was compiled, and belongs to the Brythonic branch of the Celtic family within Indo-European, distinct from the Goidelic branch that includes Irish and Scottish Gaelic.
Middle Welsh preserves substantial evidence of pre-Christian Welsh and broader British Celtic mythology, including figures such as Rhiannon, a horse-associated goddess-like character, and Arawn, king of the otherworld realm Annwn, embedded within narratives that Christian-era Welsh scribes recorded and reshaped, much as Old Irish scribes did with earlier Irish pagan tradition, producing texts that blend genuine mythological substrate material with medieval literary and Christian reworking. The Mabinogion's four core branches, along with related independently transmitted tales including Culhwch and Olwen, one of the earliest surviving Arthurian narratives, together preserve the most substantial body of medieval Welsh vernacular prose literature to survive from the period.