What or who is the Mabinogion?

Answer 1

The Mabinogion are the earliest prose stories of the literature of Britain. The stories were compiled in Middle Welsh in the 12th–13th centuries from earlier oral traditions.
The name first appears in 1795 in William Owen Pughe's translation of Pwyll in the journal Cambrian Register under the title "The Mabinogion, or Juvenile Amusements, being Ancient Welsh Romances.

Answer 2

A Welsh language collection of pre-14th century British literature
The Mabinogion is preserved in written form in the White Book of Rhydderch (1300-1325) and the Red Book of Hergest (1375-1425). Parts of the stories are thought to date from the second half of the 11th century, parts much earlier. The stories, compiled in Middle Welsh in the 12th–13th centuries from earlier oral traditions, are heroic Welsh epics and early Arthurian traditions. William Owen Pughe translated them into English before he died in 1835 but did not publish. The Mabinogion first became generally well-known in the mid 19th century, when Lady Charlotte Guest published her own translations in a bi-lingual presentation of the works, beginning 1838.