Ionas, Jonathan, Jonathas
Earl of Dorset or Dorchester under King Arthur.
He fought for Arthur in the Roman War and led half a legion of warriors at Soissons.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Roman de Brut | Wace, c. 1155
Brut | Layamon, late 12th century to mid-13th century
Alliterative Morte Arthure | c. 1400