Deep in the heart of Equestria, there lay a little hamlet called Ponyville. Ponyville never seemed to go a week without something at least a little odd happening. Whether it be a world-ending climactic battle, creatures pouring out of the nearby and accursed Everfree Forest, or a robot alien invasion (the debris from which they were still cleaning up weeks later), there always seemed to be something just around the corner for its inhabitants. Ponies living elsewhere in Equestria often wondered why the town's residents didn't simply move out or find a location less prone to such disasters. To tell the truth, many of the residents had become accustomed to such chaos and, to a certain extent, a few of them even enjoyed it a little.
Given such a propensity for attracting the odd and the citizens' desensitization to the danger of the oddities the town often attracted, it was of no surprise to anypony when a changeling was seen walking through the streets of Ponyville to the panic of absolutely nopony. Three ponies, in particular, became very overdramatic about the whole thing and fell to the ground despairing the non-existent horror of the situation. Other than those three, however, most ponies merely raised an eyebrow and went about their business. The changeling in question was rather average-looking, albeit a bit shorter than his other black-carapaced brothers and sisters. As he strode through the streets of Ponyville, he levitated a small map and notepad in front of his face in the signature sickly green aura that all changelings possess. He didn't even bother to look where he was going, eliciting memories in the townsfolk of Twilight Sparkle doing much the same with her books when she first arrived a couple years prior.
Stopping in front of a large oak tree, which had been partly hollowed out to create the Golden Oak Library, the changeling lowered his notepad, took a deep breath, and burned the map away in a flash of green fire. Trotting up to the door, he gave it three firm knocks, stepped back a bit, and waited. After a few moments, a small purple and green dragon answered the door in a bit of a huff. The dragon in question was Spike. Accounts differed as to whether he was the adoptive brother, servant, adoptive child, or something else entirely of the library's actual resident, local princess Twilight Sparkle.
"It's a public library, the door is always..." Spike stopped dead in his tracks as he realized who he was addressing, his mouth dropping open and his pupils shrinking to pinpricks. "Ch-ch-ch-ch..." The dragon began stuttering in fear.
"Hello," said the changeling in a far-too-cheery voice, "My name is Pronotum, and I am here to-"
"CHANGELING!!!" Spike dashed upstairs like a batpony out of Tartarus, leaving Pronotum to feel rather awkward at the open door. After a few moments, he began to awkwardly hoof at the ground and look around for a distraction, waiting for the dragon, or another resident, to return to the door. The inside of the library looked rather cozy, with books lining almost every wall that wasn't occupied by a window. A set of stairs leading both up to a second floor and down to a basement sat roughly opposite the door and to Pronotum's right.
Before too long, a purple Alicorn began to gingerly trot down the stairs, her head lowered and horn lit. This, needless to say, was Twilight Sparkle. She stopped and stared incredulously at Pronotum upon making eye contact with him. Twilight's eyes narrowed, and she frowned. Pronotum, oblivious to this, was merely enthused to have somepony new to speak to.
"Hello, my name is Pronotum, and I... Um... I am here to..." Pronotum began to avert his eyes, rubbing his left foreleg awkwardly. "Um, actually, could you come closer? It's awkward with this much distance between us. If... if that's fine with you?"
"Very well, changeling... but don't get any ideas." Twilight huffed. Nevertheless, she began to cautiously approach the insectile pony at her door. "So... why are you here, changeling?"
"Oh, yes, right, I was getting to that. But then your small dragon friend ran away, and talking to you from far away was awkward... um, but you were there for that last part." Pronotum awkwardly rubbed his forehoof again and took a deep breath. "A-anyways, starting over again... Hello, my name is Pronotum, and I am here to... um..."
Pronotum coughed uncomfortably and flipped through the notepad that still floated to the side of his head. Twilight eyed it with suspicion, though confusion and curiosity were beginning to break through her prior sternness. This changeling's behavior was very unlike that of the changelings Twilight had encountered previously at her brother's wedding, and a small part of her wanted to know why.
"Ah, here it is." Pronotum cleared his throat. "And I am here to Step Four: Infiltrate Ponyville and don't get caught, after which I will Step Five: Neutralize the Elements of Harmony so that I can Step Six: Begin Invasion." The changeling smiled and held out a hoof. "It is nice to meet you, and would appreciate any help with said tasks."
Twilight stared at the outstretched hoof, then at the changeling himself, then again at the hoof, trying to process what was happening. Then she began to laugh. She began to laugh very, very loudly, and very, very hard, and she dropped to the floor.
Pronotum looked on at the purple pony princess, confused. "D-did I mess up? C-can I start over if I messed up? I haven't really done this before, I'm sorry if I did something wrong."
"N-no, haha, no, i-it's just..." Twilight began to regain her composure a bit. "T-the Elements of Harmony have been gone for about a month or two now... and... and the way you're acting..." She took a deep breath and sighed. "T-thanks, I kind of needed a laugh like that. It's been a hard week."
Twilight righted herself and smiled. Of course, Pinkie and Dash would use a prank to cheer her up, she thought. There's no way a changeling would act like this anyways. "I know you two like pranks, but I must know... how did you find the illusion magic to look like a changeling without me figuring it out? Even for your pranks, this is pretty high-quality."
Pronotum stared for a few moments, then began to shake. "G-gone? C-can you get the Elements b-back?"
Twilight's smile faltered slightly and she rolled her eyes. "No, they're in the Tree of Harmony now. Remember?"
"B-but, mom s-said I had to g-g-get the Elem-m-ments. B-but there aren't a-any elem-m-ments anym-m-m-more?!"
Twilight's smile disappeared completely and she backed up a step. "Come on... it was a funny prank, and you cheered me up. You don't have to keep up the act..." Suddenly, Twilight wasn't so sure that it was an act.
Pronotum collapsed to the ground and began to cry. "Please don't be m-m-mad m-mother, I t-t-tried so h-hard..."
Finally, everything clicked.
"Sweet Celestia... you're completely serious. This isn't a prank. You're actually a changeling." She stared down at the changeling in front of her, now curled up in a ball on the floor and mumbling about being punished. To be completely honest, Twilight felt a little bad for the little guy. "Hey... calm down, um... Pronotum, was it?"
Pronotum looked up at Twilight, his face covered in tears and, disgustingly, a bit of snot. "Y-yeah..." He sniffled, which immediately led into a hiccup. This changeling was a complete and utter mess.
This changeling was also a prime opportunity for Twilight to learn more about its race. Even in a situation like this, Twilight began to pick up on the chance, and the gears in her head turned. Twilight put a hoof on the blubbering bughorse's shoulder.
"Come on, Pronotum. Let's get you inside and get some tea in your system. Then maybe we can talk about this, okay?"
"O-okay... t-that sounds nice." Pronotum gave a pitiful smile and slowly stood up, following the mare into the library. And with that, the door, enveloped in a purple aura, closed behind the pair, obscuring the scene from the dozen or so ponies who had decided to stay and watch. A few ponies, who had made bets on the situation, begrudgingly paid up for their lost bets. One by one, everypony went back to their normal lives.
Though they rarely ever saw the changeling again in town after that, nopony was surprised when Pronotum attended a wedding the following Spring. Then again, they would probably be just as jaded about it even if they hadn't seen him walking through town several months prior. After all, it would just be yet another weird thing to add to the pile, and said pile was practically an ocean by now anyway. As the Ponyvillian saying goes, 'it's just another Tuesday.'