It is important to note that "horsey" or "horse-faced" are actually insults! They usually mean that someone has extremely large nostrils or smells like manure -- which of course does not apply to any of the members of the Royal Family, or future members of the family like Kate.
I would actually describe Kate's beauty as "unicorn-y."
Some of my particularly uncultured readers may not know the difference between a horsey face and a unicorn-y face. I have prepared some helpful diagrams to educate you and get you ready so that you will not commit a faux pas in describing the Royal Couple at your wedding parties. Click to view full-size!
Horses vs. Unicorns