Detailed answer: Her eyes in S1 and early S6 used to be similar to a stallion's eyes and that never changes til Season 7 , and she was conceived as a male character
The obvious and easy answer: It's a fun headcanon, nothing more nor serious until someone makes it serious
why the fuck do people keep thinking Trixie's trans.
look at her. that's not a stallion's muzzle. if she were MtF she'd have a stallion's facial structure.
plus, "Magic Duel" features Trixie and establishes that you can't actually change mares into stallions even with magic.