Or what if Rainbow Dash turned out slow?
Or what if it turned out Luna's job was to raise and lower the sun?

I'm not advertising a backwards universe.

Ponies are usually given names that fit with their special talent, which also fits the cutie mark. But depending on when naming is done (Cutie Mark Chronicles puts it as while they're young but before the cutie mark with the addressing of Pinkie Pie), the given name may not later match the talent.

Take Apple Bloom, for instance. Her name is apple-related to keep with the rest of the Apple Family. But the thing she's shown a talent for throughout the show is construction. Her cutie mark might be something construction- or design-related and not apple-related. Her name and cutie mark, after she gains it, may not match exactly. Derpy's another odd case; I can't exactly think of an obvious connection between "Derpy" and "bubbles".

So, what happens when the name doesn't match the eventual cutie mark?

2127886
You get Derpy Hooves.

Also known as awesomeness.

2127886
the universe implodes, is what happens.

2127886
I seem to recall the fanon reason for Derpy's mark is her bubbly personality, but I could be wrong.

2127913
Again, not entirely obvious considering we haven't actually seen much of the character except in fanon.

2127886>>2127901>>2127906 1÷0

2127926
THE FUCK, PORTALZ!

Are you trying to kill us all?

2127931

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2127886 Derpy is a fan name which was adopted to canon.
But I can see the paradox you have set up.

2127901

Also known as awesomeness.

*also know as autism
there fix'd

2127949 Is it Autismᴴᴰ?

2127984 maybe

2127886

That's because Derpy's name was concocted by fans, not by the writers of the show. The fandom as a whole obviously does not think things through logically before adopting a meme.

2127886
Really to match her cutie mark Celestia should be named Solara or something like that, since Celestial refers to all the heavens and not just the sun. But yea, for the most part; their names do usually match their talent.

I'd say that the parents somehow instinctively know what to name their children. It is a magical world after all. Just every once in a while you get one that doesn't quite fit.