I rescued my dog Jack off the side of the road about 4 months ago and he's roughly 10 months now. Jack is generally calm, well behaved, great with most dogs and all kids and adults. He's lived with 2 different dogs in the 4 months since I've had him and has met/played with at least 100 different dogs during walks, at various dog parks, in pet stores, at the vet, etc. On 2 occasions Jack has shown serious aggression towards a strange dog. The 1st time I met a dog walker with a young puppy, close in size to Jack. It wasn't apparent to me what it was but something in the behavior of the puppy flipped Jack out and he started making jungle cat noises and generally acting really scary. Both dogs were on leash and the situation was quickly diffused. Fast forward a couple of months to today and Jack's 2nd incident of seemingly inexplicable scary aggression. My roommate and I brought our 2 dogs to an off leash dog park in Boston that we go to a lot. Besides our 2 dogs there were 3 others at the park; an adult bull mastiff, a 5 month old golden puppy (the target of Jack's aggression) and a yippy mid size mixed breed dog. Jack and the golden puppy were very close in size and were touching noses, greeting each other with no apparent problem but it got ugly fast. The owner of the golden grabbed Jack, separated his dog and mine then let my dog go at which point Jack did his scary dog routine and actually chased after the puppy. The golden owner basically tackled Jack and passed him on to my roommate at which point I put him on leash and hauled him out of there. If you've read this post and have any idea what might be going on, please let me know. Or, if you need any more information please ask. Also, if anyone has dealt with something like this and has advice I'd really appreciate it.