I think it means the bartender asks about horse's sadness expression

I think it's a play on words with a double meaning. If it were a person a long face would me "why are you sad, but a horse naturally has a literal long-face.

Yes, in English, having a "long face" means to be sad about something. A horse's head is long and therefore the horse has a long face. It is a play on words.

I get It.

Nope