Today Phoenix growled and snapped at our neighbours new Boston Terrier. She brought him over to show us, we met outside and I brought Phoenix with me to see him, and he did not like him at all. I have never seen this side of him, he kept trying to hide behind me, he wouldn't even look at the other dog. Phoenix has been great with all other dogs he meets. I was wondering if he sensed something about this dog??? Being territorial?? This behaviour just really surprised me. I'm not sure if the other dog is altered or not, but my husband didn't think so. Any ideas??

I tend to think dogs are like people and some of them just don't get along.  One of my RTs goes into attack mode when she sees a Shibu Inu.  She might have had a bad experience with one but she does not like that breed of dog and it's as if she wants to fight to the death.  It might have been that your dog was scared and that was his way of telling the other dog to back off.