So I've had my 7 year-old Golden Retriever for about a year now. She is very gentle and has always been playful and kind to ALL dogs. The other day we rescued a Collie mix who we found on the street. She was in desperate need of help, so we didn't have enough time to introduce the two (which I know is incorrect, but the rescue HAD to come home with us immediately). However, they were both kept separate. (Golden in the house and Collie in the Garden). The Collie is also kind, but of course, needs some time to adjust to her new surroundings. We went to take the two for a walk, so they could get to know eachother outside the house, but as the Golden walked past the Collie (from pretty far away) she ran towards her and bit her REALLY hard. We were very surprised! Our Golden Retriever is now TERRIFIED and can not even go into the garden anymore. What can I do? I wanted them to learn to be friends and live in the house together, but our Collie is showing extreme signs of aggression to the Golden, who seems to be doing nothing wrong. Please help!!