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In a fantasy world everything is possible. Though you won't see Helseth making out with his stepsister in a leather fetish movie any time soon because their characters would make such a turn of events very unlikely. He'd be tide up and she'd be biting his other ear off.
So yes it is possible that somebody took a Netch, decided to change everything characteristic about it and turn it into a Stilt Strider. He could have supersized any ordinary flea from his asscrack and be done in five minutes but lets assume there is the possibility that some crazy Telvani that doesn't have that clarity of mind.
Since it is a real possibility that 'the wizard did it', and there is no lore to counter it -it is gameplay afterall- this argument must obviously be right! Except it isn't because absence of contradiction is not proof of confirmation. Which there isn't any off, (and hence theories are incomplete, thus only rather true for a limited set of observations).
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Now for the second argument, if Netch could have evolved from Stiltstriders or viseversa, it is like saying that Men descends from Ape. Which popular memes aside, he doesn't. Men and Ape share a common ancestor.
So wrong.
But a Netch and Stiltstrider might also share a common ancestor. If not because they evolved that way, -not likely since Nirn has some 7500 years on the counter- then because they were made from the same bit from the same god that took part in the creation of Nirn.
So this about the same as saying that Men and Dinosaurs shared a common ancestor, somewhat obvious, but also some what trivial, or in other words not a discovery and dumb.