So I got this idea from that 'Rainbow factory' spinoff I never got around to write. What if one of the prominent characters (Mane 6, princess, CMC...) was actually a robot sent by a foreign nation in order to spy on Equestria?

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And what, if such instructions came into conflict with the robots' own "emotional response" subroutine?

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What if one of the prominent characters (Mane 6, princess, CMC...) was actually a robot sent by a foreign nation

Hmm. If the robot was one of the mane 6, and they were able to use the Elements of Harmony, what does that say about the robot's "personality"?

I mean, you could make this plausible, if the robot was a "sleeper agent" with no idea of its own directives until "activated". Or its directive is far more subtle, like, "just look at this and that sensitive document for half a second, and let your built-in cameras transmit it back to us". And the pony in question just sometimes feels weird "urges" to do odd things but nothing obviously malicious, never knowing she is feeding information to "the enemy". Which means it could be Pinkie Pie, with her twitches and all her randomness.

If it's one of the princesses... Okay, you'll have to explain how that one works. Celestia was replaced by a robot and nopony noticed, not even her sister? Celestia has always been a robot, and that's, why she's immortal? The Luna that came back from the moon was a robot? Ascended Twilight was a robot all along, and the real Twilight ended up somewhere else? Cadence is a robot capable of using love magic and having a foal? Flurry Heart is a robot, and Cadence didn't notice the swap when she gave birth?