1. Evan/Ifan are Anglicized versions of the Welsh names Ieuan/Iefan and hence derived from John [q.v.], from Hebrew "God has been gracious". They are not associated with Ewan or Euan. Ioannes, Iohannes, Joannes or Johannes are all Latin forms of John and its derivatives, used in old documents with appropriate Latin case endings.
2. Evan is also an Anglicized version of the Scottish Gaelic names Eghan/Eóghan/Eóghann as is Euan which we have therefore classified as a Lesser Synonym.