Lets see...
1) In most states, cats (and other pets) are treated as property. As such, you could file a complaint with your local police department for theft of property.
2) If you are considering civil action, I assume that you will want to use the Small Claims courts. If so, most states only allow those courts to consider money issues, not really the return of property. So, you will need to show the actual 'value' of the animal to prevail in court. And hopefully, the amount would be high enough to convince the 'new owner' that it is best to return the animal rather than to pay the money.