She's got a face like a slapped horse

I have never heard the expression (in the U.S.) but I love it. Here's my guess: "Horse-faced" means a long, homely face. So if the horse was slapped, it would look even worse.

Hmmm. The way in which it was said was more like "she's looking like a slapped horse at the moment". Not always, just right now. A mood. Meaning?...

Ready to bolt? Just a guess.

Most people slap horses on the rump. Like with a crop, to get them to run faster. But you can imagine slapping a horse in the face and you'd get another kind of expression, and reaction.