Believe it or not, it might be possible for your neighbor to enforce CC&Rs (Declaration of Covenants, Conditions, and Restrictions) when there is not HOA through a personal lawsuit.
It will depend entirely on your state’s rules regarding absent HOAs. Some states believe the CC&R ceased to exist when the HOA does, while others allow enforcement through private citizens.
If you feel that your disgruntled neighbor will take you to court, get in touch with the rest of your neighbors and see if they will agree to sign off on having the CC&Rs lifted. If enough people sign a notarized document and submit it to the state, you may be able to keep your koi pond.