Is this a discrimination situation?

We moved into a mobile home park in october 07. In January of 07 a rule was passed that no dogs over 40 pounds are to reside in the park. However, they let us move in with a 75 pound dog which is very well mannered 1/2 golden/1/2 Australian Shepherd. He is our sons dog and once our son moved to the country he took him. We were wanting another puppy and was blessed with a 6 week Golden Retriever. Now their saying we can’t keep him because he will be over 40 pounds. A Doberman, Grey Hound, Chocolate Lab and other over 40 pound dogs reside here and they say its because their Grandfathered in. We are facing eviction if we do not get rid of our puppy. I see no fairness in this situation and I am home with this puppy most of the time. Other rules are being broken in the park as well like motorcycles (rules state no motorcycles) & people living with family members who are not registered and back ground checked. I don’t care what everyone else is doing it is their business. I am just wondering if this is a form of discrimination. Help! Our neighbors all disagree with the parks decision too. They are witness to us keeping very good control of our pet. Please I need input! Meanwhile we are looking for a place as I am not getting rid of our Golden!
The rulel went into effect in Jan. 07 and they let us move in on Oct. 07 with our 75 pound dog. How could this be grandfathered legally?