CREATE TABLE table_26519 ("Co-king" TEXT, "Relationship to Monarch" TEXT, "Crowned" TEXT, "Co-kingship ceased" TEXT, "Reason" TEXT, "Monarch" TEXT)

Name the monarch for 13 october 1131
SELECT "Monarch" FROM table_26519 WHERE "Co-kingship ceased" = '13 October 1131'
SELECT "Monarch" FROM "table_26519" WHERE "Co-kingship ceased" = '13 October 1131'