Golden Retriever Aggression

Question
Yesterday, we received a 5 year old male Golden Retriever from a friend who is moving. While the dog was visiting, he was the friendliest dog to our children, us, and our dog. My son and i got the dog out of his pen this morning and took him for a walk, but after school and work, he changed into a mean dog. He started to growl at us when we even come near his pen, and once lunged at my husband through the fence barking and growling. We let him alone for a while. I went back later and he let me pet him through the fence, but when i would go to let him out, he would attack again. What is wrong here?

Answer
I think you accidently scared him somewhere. Just reasure him with treats and praise. Dont ever look mad or mean around him. Tell your children to run in the backyard or slowly in the house. This should temp him to chase them and play tag. And once he is used to you. Wrestle with him in a playful manner( while smiling or laughing) This will assure him that its okay even if you are handling him. If this doesnt work the last thing you could do is wait. If still no response, email me back. Hope this helped. Bye.