Pitch was being bothered by a certain pesky winter spirit, who was looking all over the caves for him.
"Hey! Piiiiiiiiiiiiiiiiiiiiiiiiiiiiiitch!"
Unfortunately, Pitch had been found before he could change out of his everyday clothes and into his robes. Jack was silently staring at him for what seemed like an age before he burst out laughing.
"PINKIE PIE!" he managed, wiping tears of laughter from his eyes. Pitch flushed as pink as his t-shirt and stood in furious mortification, waiting for Jack to leave, to get out and leave him alone, while knowing he would probably have to wait for hours and suffer for that long.
And the moral of this story is that even the Nightmare King can be a brony.