How Pterus got his cutie mark.
 

 
He and his sisters were outside playing with their parents taking care of some business just a bit away, thinking their kids would be safe in one of Canterlot Castle’s inner gardens. They were wrong. An extremist nutjob, who viewed Twilight and Luna’s marriage very disfavorably, saw them having biological foals together as a violation of the very laws of nature, a sign of degeneracy in the highest office of the land and believed it was a conspiracy to eradicate males from Equestria. So he tried to kidnap the two girls and hold them for ransom.
 
Little Pterus, who had been trying to sneak up and scare his sisters, saw the menacing stallion approaching them. While yelling for his moms, guards, anypony who help, he jumped from his hiding place to get in between the stranger and his sisters. It spooked the stallion for a moment and delayed him in his plans just long enough for the adults to run, fly and teleport in to subdue the stallion before he could run too far or hurt any of the foals.
 
In saving his little sisters, Pterus realized he rather liked the feeling of protecting somepony who couldn’t protect themselves. It felt right. He felt a purpose for himself. It wasn’t until the stallion was being hauled off and Twilight was frantically checking over the three foals to make sure they weren’t hurt that any of them noticed Pterus’ newly formed cutie mark. It was an extra special cuteceneara.