bartenders are known for their ability to listen to the problems of their customers. horses have a "long" face, but the long face is an idiom for a "sad face", so the bartender is indicating that he wants to listen to the problems of the guest, even when its a horse.

If some one has a long face it means they are upset or sad, and the satement is a joke because a horse already has a long face.

Three elements define the structure of this joke and its intention to provoke a laugh. (1) The expression "Why the long face?" is another early 20th century idiom for "You look down in the mouth." When one is sad or ruminating the facial muscles go slack and the corners of the mouth turn down. By the mid-20th century the stereotype of the white shirted bartender wielding a matching bar towel and leaning over to ask his patron "Why the long face?" had become ingrained in the popular culture. The visual cliche had been nearly forgotten when this joke emerged in the late 20th century, leading perhaps to confusion for some and smugness in others upon hearing the joke. (2) The humor is inherent in the absurdity of the play on words regarding the elongated rostrum of equine individuals, and (3) in the tandem absurdity that by now everyone else seems to have walked into a bar in a joke at one time or another, so why not a horse?

It means that the guy is vantastiic

haha..mfcharlyb,my thumbs up! I've been laughing so-o-o long! thanks for the question and the clever answers.