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The law is that the board must enforce the bylaws consistently and fairly. It is not acceptable for the board to every apply a bylaw to one person and not to another. For this reason you have a strong case to compel the board to overlook the bylaw and allow you keep your dog simply because they did that for the other owner.
I suggest you, or a lawyer acting for you, send them a letter stating that this is your view and threatening a lawsuit.
If you want to have a bylaw changed you are going to have to get 75% of the owners to vote for the bylaw change.