I have never actually seen any official material come out to say Tsils was meant to be a girl. Only rumors. Since his designer always wanted to use the name Miles Prower I find the rumor unfounded. That said Tails was only one design of Sonic's potential sidekick. So there could have been female designs that wasn't used.
Considering one of Sonic's original designs was going to be a bunny, that does make sense. I like seeing prototype designs though, and what could have been.
Apparently "bunny Sonic" would have had game play mechanics similar to Ristar. Though it's interesting because typically rabbits are associated with speed and quickness, but maybe they were trying to get away from cliches and stereotypes. A fast moving cool hedgehog with an attitude caught on and gained popularity back in the 90's.
But an origin story for Sonic's speed would be cool. We see origin stories for Shadow, and to a degree we see Knuckles' lineage. Sonic remains a mystery, as does his two tailed side kick.
I do think Tails being a kitsune makes sense, it explains why he's so intelligent and why he has a second tail.
Is there a source that implies Tails was originally going to be female?
Even if he's not a full kitsune, I think he's at least half. His father could have been a normal fox, while his mom was a kitsune.
The only thing is I don't see Tails as a trickster, he's a good natured individual. Of course, he could have been luring Sonic into a trap this whole time, who knows?
I can't recall where I read it originally, but as I understand it he was originally going to be female. Apparently SEGA of America thought it was inappropriate that a young girl follow Sonic around, and then SoJ was basically already doing the same thing. As for the trickster bit, you can be tricky and still be a good guy. Think EVERY rogue in any RPG. But I could go with the Kitsune thing, even if he isn't aware of it. But mutation is probably easier to believe since Tails was a creation of SoA.
Good point, and when you consider any time Tails gets "killed" in the game, he always comes flying back. Perhaps he's very good at faking his death only to come back unscathed just to trick Eggman. I will likely head canon he's at least half kitsune, but isn't aware of that heritage. I guess I'm not crazy about the mutation idea, but then again I find Japanese lore fascinating and feel like it fits.