There are 4 "Boar's Head" songs or carols in A. H. Robbins' 'Secular Lyrics of the 14th and 15th Centuries', and 3 in A. H. Bullen's 'Carols and Poems', and one in Chappell's PMOT. See also 'The Oxford Book of Carols'. The head had to be that of a wild boar, which has been extinct in England since the 17th century, so all such songs are presumeably older.