Green Knights are enemies seen only in the Haunted Castle.
They behave aggressively when the Spelunker is seen, charging at them to deal damage. They will run off ledges and do not stop charging until they take damage.
Due to the armor they wear, they cannot jump but are immune to damage from being whipped and stomped (unless the Spike Shoes are worn), so the best way of dealing with them is to throw objects at them or use weapons.
When they take damage, the suit of armor breaks to reveal an ordinary Caveman, with two hit points remaining.
The Cavemen inside the suits are a good source of corpses to sacrifice on the special Altar that always generates at the bottom of the Haunted Castle.
It is a mystery whether the cavemen inside the suits are falsely masquerading as evil spirits or whether they are trapped inside the haunted armor, used as physical hosts for possession.
This enemy and its resistance to many forms of damage is likely a reference to the invincible "Green Knight" of Arthurian Legend. According to the 14th century poem Sir Gawain and the Green Knight, a mysterious green knight challenges a one of the Knights of The Round Table to attack him with his own axe, on the condition that the green knight would return the blow a year later. Upon being decapitated, the Green Knight simply reattaches his own head and tells the shocked Round Table Knight where they will meet the next year.
Alternatively, this enemy may be a reference to the game Super Ghouls and Ghosts. In that game the player can acquire a green suit of armor and, upon taking damage, the armor is lost - leaving the player in their underclothes, much like the cave men.