Whether your furry friend needs a new outfit for a special occasion such as a birthday, or has to conform to the dress code of your wedding, believe it or not (I know you believe it!) there are plenty of websites that offer formal clothing for dogs of all sizes.
Of course, if you decide to try this, you should probably make sure this will be accepted at your individual wedding location. Some chapels, churches, and other wedding locations may not be as thrilled by the idea of animals in the aisle as you are. Once you have received the go ahead from the venue, you can proceed to begin your search for formal wear and plan your pet's role in your special day.
I suppose the worst thing that could happen would go something like this: "Great! A new chew toy! How do I get this off? Grrrr...Almost got it...Ahhhhh! There we go! Hmph! That was a weird way to give me a treat."
My West Highland Terrier loves to dress up, and gets so excited when he sees his sweaters come out for Christmas. He doesn't have any pending wedding invites at the moment, but he does have a birthday party to go to in June, perhaps I can get him a tux for that! I particularly love the hat.
One more word of caution: after the wedding is over, you may want to hold on to Fido's tux because once all the girl's on the block see him looking so dapper, they will all be sending their humans scurrying for a doggy wedding gown so he can stand at the altar with them...but then we've got that covered too!

