The mosaic is dated from 1163, at a time when the cathedral was in the Norman Principality of Taranto. One portion of the mosaic depicts King Arthur:
Arthur in the Otranto mosaic. A Caper in Salento (capersalento.blogspot.com)
The mosaic portrays a leopard confronting a rider and obstructing his path to the Tree of Life. A second leopard attacks the unseated rider. Two yellow leopards on a red background is the emblem of Normandy. This was the coat of arms of the Plantagenet king Henry II (1154 to 1189). England’s royal arms, with an extra cat, derives from Henry’s son, Richard the Lionheart. The imagery is one of the Normans defeating the English, hence the leopard mauling the dislodged rider.
Why Arthur? The House of Godwin may well had adopted him. A number of manuscripts, including Harl. 2414 which describes him as “iarll Kernyw”, show Godwin, Earl of Wessex, having Cornish ancestry, see Coliavus. As pointed out in The Bayeux tapestry and the draco standards, it is likely that one of the standards portrayed on the Bayeux tapestry is of Cornish and not Saxon origin.
The writing “Rex Arturus” is referring to the figure hovering to the right of the picture, not the rider as is usually thought. The text is closer to him than the mounted figure. The reason that “Rex” became associated with the rider is because of his crown, but this was a late additon in a repair to the mosaic, see image:
Drawing by Aubin Louis Millin of Otranto mosaic before restoration. A Caper in Salento (capersalento.blogspot.com)
It shows areas of damage and the rider wearing a cap, but not a crown. The nature of the headgear is confirmed by the image of the rider where he is being bitten by the leopard. It appears that Arthur is in spirit form, long since dead. This may have been a Norman counter to the Breton/Cornish belief that Arthur was asleep and would rise again when needed.
The figure who is riding the goat is a representation of Harold Godwinson. He is being ridiculed with an imagery showing him going to battle riding a goat and inadequately dressed for the conflict. Arthur has his hands raised to his face, as if in horror, thus suggesting his agreement with the Normans that Harold was a false claimant to the throne.
The whole scheme being placed between the Expulsion from the Garden of Eden and the Cain and Abel story is to suggest the result of Hastings was an act of God, whose hand is depicted above. Harold’s location on the mosaic is significant. Cain and Abel are relevant as a case of treachery. It is claimed that when Harold ended up in Normandy he had taken an oath of fealty to William and then sworn on sacred relics. Likewise, Adam and Eve had broken a covenant with God. Note, the rider has his arm raised as does one of the figures expelled from the Tree of Life.
The Vita Haroldi of Waltham Abbey claims Harold survived the battle and eventually lived his life as a hermit in a cave. Although this may not be historical, it might explain the garb he is portrayed as wearing.