Matthew Poole and continuators on Psalms 68:21

21 But God shall wound the head of his enemies, and the hairy scalp of such a one as goeth on still in his trespasses .

Either 1. The political Head, their Ruler or Rulers, the devil, or other wicked Christians, or rather 2. The natural Head, as appears from the following expression added to explain this, and the hairy scalp: And he speaks of the Heads or hairy Scalps not of one, but of all his Enemies, the singular number being put for the plural, than which nothing is more frequent.

(i. e.) His most fierce and terrible Enemies. For in ancient times many people used to wear long and shaggy hair, that their looks might be more terrible to their Enemies.

Of those who persist in their enmity and rebellion against him. Whereby he opens a door of hope and mercy to his very Enemies, if they return and submit themselves to him.