What happens when you fuse a part of Nightmare Moon’s powers, and the bezerking fury of King Sombra?  
You got a werepony…
 
Every full-moon, Mane has a 1/5 chance of turning into one, and while the season of full-moon is active, the curse while be always active, until the moon shows up in another phase.  
The reason is due to the great friendship between Nightmare Moon and King Sombra, and for shit’s and giggles, they decide to put a curse in Mane.  
Not to mention, there weren’t many wereponys in that age, so It was like a rebirth…
 
He already caused waves of destruction in zones near and inside of Ponyville Itself, but one night was marked and called by “Night’s Revenge”, where Mane attacked the village 3 times in the same night.
 
In the last attack, the Royal Guard was called to experience the event, and this time, commanded by the Captain Shining Armor, one of Mane’s enemies.  
They succeeded in capturing him, but didn’t turn out very well.  
When Shining Armor turned his back to see the cause of the uproar, there was a guard with his guts outside, and by his side, his comrade being sliced alive. And inside the broken jail… a rope all broken…
 
According to the stories, only one somepony can calm down the beast inside the mind of a werepony..