Disclaimer:

I am not Leslie Charteris. I do not own 'The Saint'. I do not own Hasbro nor My Little Pony: Friendship is Magic nor related spinoff products franchises either.

Note:

Experimental crossover Alternate Universe piece, currently a one-shot, where Simon Templar (of the original 'The Saint' books series) ends up in Equestria somewhere round about the time that Sunset Shimmer was in the original series starting to get the 'big ideas' that would have resulted in her departure from Canterlot (Equestria) for Canterlot High (a human world).

Further Note:

No I do not have a clue what is going on with the formatting on this website. I keep trying to insert spaces to properly indent paragraphs and new sentences, and the website keeps stripping them out.

Sunset Shimmer (Canterlot)
Sunset Shimmer stared hungrily into the mirror.
And then the surface of the mirror rippled, and crackled, and sparked into life, and Sunset Shimmer took a few hasty steps backwards from it.
And a pony emerged from the mirror. He was sky blue in skin colour, with the bluest of blue eyes, and he was an adult unicorn with a long golden mane and tail. And the cutie mark on his flanks was some kind of weird stick-figure with a ring of some kind drawn above its head.
For a few moments, he stood upright on his hind legs, then dropped down onto all four hooves as the mirror crackled behind him and died.
He turned his head, this way and then that, inspecting himself, then rotated around, to examine himself in the mirror.
And then he turned to face Sunset.
"I say, hullo. Do I look like some kind of equine to you? Specifically like some kind of unicorn?"
Sunset blinked rapidly. She had come down here to study the mirror, and in pursuit of dreams of power, but the day was taking a rapid turn for the bizarre.
"Yes." Sunset said. "You look like a unicorn." She had wanted to preface the 'unicorn' with the word 'normal', but the adjective had choked in her throat. There was something about this unicorn which was anything other than normal.
"Right. You're going to tell me next that this place isn't 'Earth', aren't you?" he enquired.
"This is Canterlot. In Equestria." Sunset said, thinking rapidly. The mirror was supposed to show not just possibilities, but other worlds. Had this unicorn come through from somewhere else? Another thought occurred to her. "There was an old winter story, where the earth-ponies wanted to called the land 'Earth', I think, but it ended up as 'Equestria' instead."
"Earth-ponies. Unicorns. Equines. Land called Equestria. Well that makes as much sense as anything else in the past seventy-two hours." the strange unicorn said, and nodded to himself. "Well: I'm Simon. Simon Templar. Known as a good many other things to a great many other people, many such things not complimentary, but let's not bother ourselves with those for now. I was busy rescuing some friends, but we became separated by a rockfall, and they should be… well, hopefully okay now… and I'm obviously here. Canterlot. In Equestria. And a unicorn."
He moved experimentally, and did a little jig, as if familiarising himself with his body, then did a sort of whole-body stretch and roll.
"Okay: unicorn. Got the hang of that for now." he resumed. "Who's in charge here?"
"That would be me." Princess Celestia said, entering the chamber. She sounded a mixture of concerned, angry, and confused, some of it probably at discovering Sunset in this room in front of this mirror.
Simon Templar surveyed Princess Celestia, unintimidated.
"And you would be?" he enquired. "Humour the question, if you would; I just came out of that mirror, from somewhere else where I was distinctly less equine in shape, and knowledge of unicorns and matters Equestrian very vague."
"Princess Celestia. Sole Ruler of Equestria." the Princess regarded Simon Templar coolly.
"Right: and as I was just explaining to the charming unicorn here, I am Simon Templar." Simon continued. "I missed your name, if you gave it, by the way, Miss?"
"Oh: Sunset Shimmer." Sunset said.
"Sunset is one of my pupils and should not be here." Princess Celestia said.
"Right: but it's a good thing that she was here, since she was able to greet me when I emerged and to stop me from just wandering around, poking at things and hoping for answers." Simon replied. "I'm guessing, if this is a world with unicorns – and whatever kind of winged-super-unicorn you look to be, Princess – that just wandering around poking at things is not a good idea? Especially not if this is the kind of room which is normally 'off-limits'?"
There was a moment's pause, and – for a moment – Sunset had the distinction of seeing Princess Celestia with something like a look of horror flicker in her eyes for just that briefest of moments, as she glanced around the chamber.
"So: let bygones be bygones on that count?" Simon continued.
"Maybe." Princess Celestia conceded. "But: if you are a visitor to our world, Simon Templar, you will clearly need some things explaining to you and will have many questions, no doubt. Sunset: you have the rest of the day off from your studies. You will instead show Simon Templar around Canterlot, and answer any questions that he has. And then you will both report to me in the library at midnight, and we shall discuss things further."

"Thanks." Sunset said, as she exited the castle with Simon, feeling the strange sensation of gratitude in her heart for the first time in quite a while." I could have gotten in serious trouble back there. In fact I still may get in some trouble."
"Think nothing of it. Rescuing damsels from distress is all part of the service." Simon said. "And besides, without any more immediate answers available, I might just have wandered around and poked if you hadn't been there, so consider it a mutual rescue."
She actually found herself giggling, for a moment, at his manner.
"So where you come from on 'Earth'? What's it like?" she asked.
"Bipedal humanoids called 'humans' everywhere. I was one. If you know what a bipedal humanoid is like. Very smoky world. A lot more burning of things," he glanced around, "to make power to get things done and to make things work than you do here, by the look of it."
"Power?" Sunset asked.
Something in her tone made him give her a curious look.
"Steam power. Like boiling a kettle, and using that to push things. Or oil or petrol in an engine to move pistons and drive vehicles. I had a most beautiful Hirondel. And sometimes motive power transformed via very complicated mechanisms involving a lot of wire and very big magnets into electrical force. And sometimes burning coal to make gas which is easier to pipe around for cooking and lighting than it is to shift wagons of coal. From the lights back there in the castle, it looks to me like you already do a lot of the things we did in different ways."
"Oh." said Sunset, slightly disappointed. "We have some steam for things like the Friendship Express, and the lights back there in the castle were candles or ordinary magic."
"Now where I come from the words 'magic' and 'ordinary' do not go together." Simon said. "So tell me, Miss Shimmer – or do you prefer Sunset? – all about your magic?"
"Oh, Sunset, please." she said, and was sure that she was colouring slightly. He just had this kind of natural grace and charm, which even after less than fifteen minute's acquaintance was working away upon her.
And she proceeded to give him a brief explanation of magic and how it was something that unicorns (and alicorns, which she explained that Princess Celestia was) and some other creatures could do.
And he enquired regarding telekinesis, which she gave him a quick demonstration of, and then he tried it, and if he was a first-time-ever user, he managed to get the hang of it remarkably fast.
"Well, that certainly seems useful." he said in a thoughtful tone of voice, as if he were already running through all the things that it might be possible to do with it in his head, including several which she had probably never even considered.
She felt a thrill run down her spine.

Princess Celestia (Canterlot)
Princess Celestia had sent her phoenix to keep an eye on her suddenly somewhat-more-dubious-seeming-than-she-had-believed top student, and the new unicorn pony in town.
Philomena had been doing quite well until Simon Templar had unexpectedly asked Sunset for a demonstration of 'teleportation', at which point the pair had vanished in a puff of magic, and Philomena had lost track of them – at which point Philomena had had to return to Celestia to report.
Simon Templar had a ridiculous air of self-assurance, and ability to adapt to the unexpected, it seemed, and there was something about him which put Celestia in mind of a considerably more refined and much, much, more dangerous version of the airship sky-pirates of some of the southern cities beyond Equestria.
If he could be proven to be relatively benign, then he was exactly the sort of guest that Celestia would want at a Grand Galloping Gala, to upset and bewilder all the fuddy-duddies and stuffed tuxedoes of Canterlot high society.
But there was that question mark over just how benign Simon Templar might actually be? Including the way that given the time of the alarm sounding which had brought Celestia hurrying to the vault where that mirror was stored, he could have only had a few minutes to get to know Sunset Shimmer, and in that time had already decided upon Celestia's arrival to do his best to avert her wrath from her wayward student, who had been interfering with things beyond her wit to meddle with and blundering into all kinds of possible literal or moral danger.
And yet… Sunset had responded to that act by Simon Templar, and was apparently connecting with him in a way which Celestia had never seen her pupil connect with anyone else. Sunset was a filly with far too few friends, and far too many 'acquaintances of convenience'.
The hours ticked by and Celestia agonised.
They were both in the library a full hour before midnight, looking through books whilst they waited, and Celestia had ample opportunity to become aware that Simon Templar had apparently picked up the basics of unicorn telekinesis remarkably quickly if he had ever been anything other than a unicorn, as Celestia herself had heard him hint at when he'd arrived and Philomena had overheard him subsequently claim to Sunset.
Celestia nevertheless put off her own appearance until the dot of midnight, and then entered.
"The mirror thing." Simon Templar said, dipping his head to Celestia."Does your majesty believe that she might be able to return me to the world from which I arrived any time soon?"
There was a curious expression of anticipation and dread on Sunset's face, which turned to relief, as Celestia answered.
"No, Simon Templar. It is a device which I do not myself understand, and I do not believe that there is the remotest hope that I could reliably return you to any world which you came from – nor any world guaranteed to be remotely like it – any time soon."
"So I'm stuck here for the now." Simon said, a statement, not a question.
"I regret that that is the case." Princess Celestia said. She had been also considering other matters in the long hours since Simon Templar's arrival. "However: I am prepared to pay you to work for me and to send you as an envoy and observer to a place called 'Klugetown', in this world but beyond Equestria's borders. I will of course need to select a unicorn from the school here to accompany you and assist you if you are interested in this work…"

Notes:

Not much to say, since this is an experimental piece posted here to see what the audience is like. Slightly more of it (the variations resulting during events equivalent to 'The Return of Harmony') as of the original time of these notes in August of 2022 has been posted over in one of the Pony fandom threads on Alternate History dot com.

I figure that Simon Templar is one of those characters whose reaction to going through a dimensional portal and being turned into a unicorn would be: 'well that's new and interesting... so: moving forward...'

Simon Templar is Simon Templar, so a body-count of villains (mostly killing each other) is to be expected in Klugetown following his being introduced into that environment. As far as I understand it slavers operate in Klugetown, which I do not see going down well with Simon Templar.

Sunset Shimmer is assumed to be one of those characters who is a potential villain/actual villain capable of being reformed by Simon Templar's personality and charisma. He pulls it off occasionally in the original book series.