From what I understand Rio attacked a person who turned out to be the original owner 2 years ago. The original owner now wants the dog back 2 years AFTER the adoption was finalized. The original owner is taking you to court or just claiming to do so??? Don't put the eggs Before the basket.
First this is to be handled 2 ways. First, the dog attack would be handled in the criminal court system. This is where proof of the dog is gentle and had no prior attack record. Evidence of previous abuse would need to be brought. Pictures or training records would be good. Signed Adoption papers may be needed to show ownership. Also any awards or newpaper articles on the dog should be brought as evidence.
The person attacked was it on your property? or in public? This determines if he was an "invitee" to your property. This then would be covered under your homeowner's policy for the dog bite and hospital cost. If it was in public then there are other factors involved such as adequate control measures etc...
The Second part is Civil. This is Small claims court. This is damages under $3000 depending on which state you live in. This is when determination of ownership should be resolved. You have the proof. Unless he has the parent dog I don't see where he has claim to this animal.
Please DO NOT try to do this OUT OF COURT!!! You could get hurt more than you should. Educate yourself to protect yourself. Good Luck and God Bless Rio!!!