A crucial issue here is whether you can prove who has the dog. If you cannot, then you have no viable legal recourse. However, if you can prove who has the dog, then you have a couple options.
Pursuing the matter criminally would be the easiest route for you. If the police can make an arrest, this person can be prosecuted for the theft and, of course, required to return the dog. This would cost you no money and require very little effort on your part.
If the police do not make an arrest, then you might consider initiating a civil lawsuit. This method would require some costs, such as filing fees and the hiring of an attorney. It would also require more effort on your part as you would need to make court appearances and/or meet with your attorney from time to time.
Please let me know if anything requires clarification.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.