Equestria is a fantasy universe full of magic, akin to one you might find in a tabletop Roleplaying Game. So it stands to reason that any humans that might exist would follow this setting, and thus be the sort of beings who would raise armies of the dead, bind extraplanar spirits to their will and slay dragons.
 
In my conception these beings are as to Equestria as Cthulu and the various other old ones are are to any Lovecraftian setting. These are Eldritch horrors who by some happenstance of fate entered into hibernation deep beneath the earth.
 
Perhaps their destructive, unnatural magics rendered their world dead and motionless…
 
The clouds stopped and plants refused to grow as the soil grew barren, even the sun and moon came to be sluggish and threatened to forever still. For this reason the Humans bred various nature spirits to nurse their world back to health while they awaited this grand tasks completion in mystical slumber that will end once the winds blow on their own and the grass grows as wild as the Everfree, the celestial bodies plotting their own course through the sky.
 
On this day they will awaken to retake what is theirs, regardless of what the descendants of their chosen caretakers might think of this. The world recovers even now, as seen in the Everfree’s growth and soon they shall awaken…
 
And when they do none shall stand against the onslaught of their martial prowess, skills learned from a world that would make the Everfree Forest seem tame by comparison…