Help! What does this English joke mean?

A man walked into a cowboy bar and ordered a beer that President Clinton drank on TV. After a few sips, he looked up at the TV and said to himself, "This is the xxx of the biggest horse I have ever seen." A customer deep in the bar immediately stood up, walked up to him and hit him.

A few minutes later, just as the man finished drinking, Hillary Clinton appeared on TV. The man said, "She is also the xxx of the horse." A customer on the other side of the bar quickly stood up, walked up to him and knocked him down under the stool. "Damn it," the man said and stood up. "This must be Clinton's hometown!"

"No, this is the hometown of horses." The bartender replied.