A man walks into a bar and orders three drinks.
A guy walks into a bar, and things happen when he takes two frogs and a hamster out of his pocket.
A dog walks into a bar and orders a martini.
What's So Funny?
This joke relies on the mismatch of intentions. When the bartender asks his question, his focus (and surprise) is on seeing a DOG ordering martinis. When the dog answers, its focus is on the terribly high PRICES of the drinks. Humor often relies on switching expectations between the beginning of the joke and the punchline.