Who is he?

The third ruler of Equestria is known to those who understand, but are you one of them? This passage is your test, who is the third ruler?

"'Sup, plant boy?" A mocking voice coos at him, it's tone low and sinister as it speaks. Looking up at the form the third ruler is standing over him, his serpent-like furred body allowing him to lean into the stallion's eyes. Quickly striking out with a hoof, Green thumb makes for the door out of the school, only to find it blocked by standing, rattling skeletons.