In true groaner tradition:
A man goes into a pub with his Labrador, makes his way to the bar and asks for a drink. Seeing the dog, the landlord objects, "I'm sorry sir, you can't bring that dog in here. You'll have to leave him outside or leave immediately."
The man, without hesitation, says, "Please, I'm blind. This is my guide dog"
"Sorry mate", the landlord apologises ashamedly, "Here, the first one's on the house. Enjoy.".
The man takes his drink and goes to a table near the door.
Another man walks in with a Chihuahua. The first man sees him and warns, "You can't bring that dog in here unless you tell him it's a guide dog."
The second man graciously thanks the first man and continues to the bar. He asks for a drink. The landlord says "Sorry sir, you can't bring that dog in here."
On cue, the second man responds, "This is my guide dog."
Shaking his head, the landlord replies "Erm, no, I don't think so. Since when did they start using Chiwauas as guide dogs?!"
The man pulls an incredulous face and replies "What?!?! They gave me a Chihuahua?!?"