I honestly think that there may not be a human twi.
Why, you ask?
Here, let me explain. You see, if there was a human Twilight, she would go to school like the rest of the gang, right so far? Okay. So if she did go to school, then when the gang saw Pony Twi (as a human) the first time, they would be like, "Oh hi Twilight, oh is there a second one of you right there? (which would be the real Human Twilight.) Yeah, that didn't happen. So, if there was a Human Twilight, why wouldn't she be at school? DUN DUN DUNNNNNNN!!!!!!!!!!!!!