I LOVE crazy hats. I have a Koala hat and a rainbow hat and a huge brim sunhat and and and lots of hats.
And I LOVE these. How freaking adorable! Are they lined? If not, have you considered lining them with fleece or something? That would be a lovely, lovely addition (yarn gets itchy on my poor head, from experience!).
They are so cute, though. I think they could use ears, I imagine if you knit (or crochet?) an oval shape and curl it around and then attach it it would be a passable horse ear, if you get what I'm saying. I think your mom is right though. It wouldn't be excessive with the horn, though!
I really like your bridle earflap idea, though I don't know that it necessarily comes across. The design is great as-is, but I don't know that "bridle" was the first thing I thought when I saw it. Don't change the design, though. It looks fantastic.
If you are planning on selling though, a mannequin is a must-have for presentation. These are unique, so I'm sure you will find a welcoming niche, but a mannequin would really finish it off!
Best of luck! These are rockin'.