1902 illustration of Mordred, by H.J. Ford
Mabon is the Welsh form of Maponos, which derives from Celtic root •makwos (son) and the diminutive or Divine suffix -on. Thus, it means “great son.” Mabon appears in the Welsh tale Culhwch and Olwen, where he’s a prisoner freed by King Arthur’s warriors to help with hunting a boar. In Ulrich von Zatzikhoven’s epic Lanzelot, the name is rendered in the Anglo–Norman form Mabuz.
Maelwys, Melwas, or Moloas may mean “noble pig,” “prince of death,” or “young prince.” He kidnaps Queen Guinevere and only is prevailed upon to finally release her when St. Gildas and the Abbot of Glastonbury get involved.
Maleagant also kidnaps Guinevere. Other forms of the name include Melwas, Meleagent, Meliagant, Meliagaunt, Meligaunt, Meliaganz, Meliagrance, and Mellegrans.
Meliodas is Tristan’s father.
Merlin comes from the Welsh name Myrddin, which probably ultimately derives from Moridunum, a Romano–British settlement, and the Celtic roots •mori (sea) and *dūnom (hill fort, rampart). Geoffrey of Monmouth probably chose to refer to this legendary wizard by the Latinised name Merlinus because Merdinus was dangerously close to the French word merde (a coarse word for excrement).
The Beguiling of Merlin (1873–74), by Edward Burne-Jones
Mordred comes from the Welsh name Medraut, which in turn possibly derives from the Latin word moderatus (moderated, controlled). Other forms include Medrod and Modred. In some stories, he’s King Arthur’s bastard son; in others, Arthur’s nephew. Mordred was first portrayed as a traitor in Geoffrey of Monmouth’s chronicle, where he seduces Queen Guinevere, declares himself king, and wages a deadly battle.
Morholt is Isolde’s brother or uncle.
Morien is the son of Sir Aglovale and a Moorish princess.
Orguelleus means “proud, orgulous,” from the Anglo–French word orguil (pride). Several Arthurian characters have this name.
Owain comes from an Old Welsh name variously spelt Ougein and Eugein, and thus may ultimately derive from the Greek name Eugenios (well-born). It may also have the Celtic roots *owi- (sheep), *awi- (desire), or *wesu- (good), plus the Old Welsh suffix gen (born of). Owain is a Knight of the Round Table, and usually written as the son of King Urien of Gore and the philandering husband of Laudine, the Lady of the Fountain. He’s one of the Arthurian characters who actually existed.
Sir Pellias, The Gentle Knight (1903), by Howard Pyle
Palamedes may derive from the Greek roots palai (long ago, in days of yore) and medos (schemes, plans). He’s a Saracen Knight of the Round Table.
Pelleas, or Pellias, may come from the Greek name Peleus and the root pelos (clay). He’s a Knight of the Round Table and the husband of Nimue, the Lady of the Lake.
Pellehan, or Pellam, possibly derives from the Welsh name Beli Hen (Beli the Old). Beli may be a nickname for Belenus, which comes from the Gaulish name Belenos or Belinos and possibly the Indo–European root *bhel- (brilliant, bright) or *bel- (strong). Pellehan is the keeper of the Holy Grail.
Pelles is the son of Pellehan and the father of Elaine.
Pellinore may derive from the Welsh name Beli Mawr (Beli the Great). He’s a son of Pellehan, King of Listenois, and eventually part of King Arthur’s court.
The Temptation of Sir Percival (1894), by Arthur Hacker
Perceval was created by 12th century French poet Chrétien de Troyes, who may have based it on the Old French phrase perce val (pierce the valley) or the Welsh name Peredur. If it’s Welsh in origin, it may mean “hard spears,” from the roots peri (spears) and duri (hard, steel). Other forms include Parsifal and Parzifal (German), Perchéval (Picard), Percevelle, and Percival. Perceval is a Knight of the Round Table who achieves his quest for the Holy Grail.
Peredur (etymology above) is the 14th century Welsh equivalent of Perceval.
Rhun may derive from Proto–Celtic root *roino– (plain, hill) or *rnf (magic, secret). In the 1380s Red Book of Hergest, a story about time travelling in a dream, Rhun appears as a counsellor to King Arthur when 24 knights seek to make peace.
Rivalen is the German form of the Old Welsh name Rhiwallon, which in turn comes from the Old Celtic name *Rigovellaunos. It may mean “most kingly” or “lord ruler,” from roots rhi and gwallon. This is the name of Tristan’s father.
Onomastics Outside the Box 
And now I know why Merlin is Merlin!
“Merlin comes from the Welsh name Myrddin, which probably ultimately derives from Moridunum, a Romano–British settlement, and the Celtic roots •mori (sea) and *dūnom (hill fort, rampart). Geoffrey of Monmouth probably chose to refer to this legendary wizard by the Latinised name Merlinus because Merdinus was dangerously close to the French word merde (a coarse word for excrement).”
With the sea and the hills.
I don’t know that I would have called Mordred particularly moderate – but that IS his name… [or the meaning thereof!]
“Mordred comes from the Welsh name Medraut, which in turn possibly derives from the Latin word moderatus (moderated, controlled). Other forms include Medrod and Modred. In some stories, he’s King Arthur’s bastard son; in others, Arthur’s nephew. Mordred was first portrayed as a traitor in Geoffrey of Monmouth’s chronicle, where he seduces Queen Guinevere, declares himself king, and wages a deadly battle.”
[in most of the stories I have read he is the bastard son – and I do have some experience of Mordred-as-traitor].
And this Rhun seems to be magical:
“Rhun may derive from Proto–Celtic root *roino– (plain, hill) or *rnf (magic, secret). In the 1380s Red Book of Hergest, a story about time travelling in a dream, Rhun appears as a counsellor to King Arthur when 24 knights seek to make peace.”
And seeing ALL the Percival variations in front of me – as well as the Pierce the Valley definition/etymology…
“Perceval was created by 12th century French poet Chrétien de Troyes, who may have based it on the Old French phrase perce val (pierce the valley) or the Welsh name Peredur. If it’s Welsh in origin, it may mean “hard spears,” from the roots peri (spears) and duri (hard, steel). Other forms include Parsifal and Parzifal (German), Perchéval (Picard), Percevelle, and Percival. Perceval is a Knight of the Round Table who achieves his quest for the Holy Grail.”
[that “Hard Steel Spears” is a help as well].