Technically Helovia asks for your gender, which is different from your sex (as you know). HOWEVER, I don't think that field was created with the wider philosophical/societal implications in mind. Really, we're asking for sex in that field.
For simplicity, I'd say to put stallion, and then the very large note. But of course that goes against basically everything that trans m/w fight for, and because that designation really only matters for breeding, you might argue that it shouldn't matter at all.
SO. If for LGBTQ+ support reasons, you'd like to put mare, despite the sex being a stallion, let me know and I'll chat with the rest of the team. If you don't want to confuse the community, I'd put stallion in the gender field.
Let me know!