The Welsh tale Culhwch and Olwen is usually considered the earliest extant complete Arthurian tale, dating to the 11th century, and including traditions that probably predate that significantly. As a result, it lacks the later French Romance influence that permeates other early Welsh prose, like Gereint and Enid. In essence, it's the "original" King Arthur, as close as we can get.

One of the coolest parts of Culhwch and Olwen is how it contains lists of names associated with King Arthur's court--the original Knights of the Round Table, you could say, although the concept of the "Round Table" had not yet been invented in Arthurian legend, and the list extends beyond knights. Morris Collins has an extraordinary, complete, annotated list of the court in Culhwch and Olwen. Some cursory observations:

Many famous Arthurian knights are missing. Only Sir Bedivere (as "Bedwyr") and Sir Kay (as "Cai") seem to have remained famous without much alteration through the years, from Culhwch onto Mallory. Sir Gawain is significantly present as "Gwalchmei mab Gwyar," a form which bears little resemblance to his later tradition, and Sir Tristan (as "Drustwrn Hayarn") may be mentioned. Otherwise, if you have a favorite knight, they're probably not there. No Gareth, no Percival, no Galahad, and certainly no Lancelot.

There are some historic people included. There are several historic members of Dumnonian royalty included, most notably the entire house of Erbin, the historic King of Dumnonia in the late 6th century: Gereint son of Erbin, Ermid son of Erbin, Dywel son of Erbin, Cadwy son of Gereint, Gwynn son of Ermid, and others. Erbin's historic uncle, Tristan, may also be included, as mentioned earlier. Undoubtedly, the Welsh remembered this particular court fondly, as a powerful but now lost ally, as can be seen in many other works, like Y Gododdin and Gereint son of Erbin. Historic kings of the Old North, also lost allies, get a mention as well, as do more recent Breton royalty. Some invented French kings are included, probably farcically, or as wishful thinking. Not surprisingly, there are no Angles or Saxons, but the Osla Gyllellvawr character would later be confused with Offa of Mercia, and turned into an Angle in folklore. There are surprisingly few historic Welsh individuals, probably due to the fact that those hearing the tale would be more familiar with Welsh individuals, and would have difficulty suspending disbelief if they recognized a name that certainly did not belong in Arthur's court.

There are some pagan gods. Reflecting the most popular Celtic gods at the time of the Christianization of Wales, some mentioned in this court list have their origins in Celtic paganism. Lloch Llawwynnyawc is probably one of multiple forms of the god Lugus seen in the Mabinogion. Manawydan son of Llyr is undoubtedly a sea god, as is his father, Llyr (see The legend of the Men Scryfa for another run-in I've had with Llyr). Llud Llaw Ereint is probably derived from Nodens, a god of the sea and healing. The Lord of the Underworld, Gwyn ap Nudd, also makes an appearance, as does the god known as Aeternus.

There is a lot of nonsense.
Just read through the list--there are some weird ones. It's no wonder "Llwng" was never transformed into a "Sir Llwng" of legend.