Who do you headcanon as a-spec? Here's my list:
Big Macintosh, Scootaloo, Sunset Shimmer, Octavia, and Zecora are all ace.
Silver Spoon is aromantic.
Lyra and Screwball are aro-ace.
Doctor Whooves is asexual and agender, which I mostly headcanon because of all the associated evidence in Doctor Who. I won't talk about that here because... there's a lot of it. If you want to know more about my thoughts on Gallifreyan genders and pronouns, I am completely willing to babble on for hours with lots of citations and sidetracks about the Time War.
Snails hits the trifecta; ace, aro, and agendered.
Fluttershy is demi.
Anyway, those are my random thoughts and feelings about horses. What are yours?

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I headcanon Luna as ace, but not aro, and have written extensively about her (in a happy relationship too).

I have Sombra as demi grayro, and he's also in a lot I write.

Rainbow Dash I've always thought of as aro ace.

I think I've had some ace background ponies and minor OCs too.

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Hc Celly as Demi, and Luna greyace

Twi hella aro-ace

and Pharynx aro but not ace