Dog Aggression Problem

JP08

Little bit of an odd one.

My parents have two dogs, total opposites. One is an extremely placid terrier - just wants to sit on you lap and sleep. Does nothing terrier like. Doesn't chase anything, doesn't dig, plods around. Almost acts like an old dog, despite being under 5. The other is about a year younger and the exact opposite. A complete dynamo that will fetch ball until it flops in a heap, rest for 5 mins then play on for another hour or two. All day. With anyone who is on hand. Pushing the ball at them and whining etc. You get the picture.

The problem is that the excitable one has recently taken to turning on the placid one (who doesn't fight back) under excitable situations - eg a stranger appearing at the gate. Manic mutt will hurtle up to the gate barking and jumping etc - being total terrier. Placid one will wander up. And possibly get attacked. Real knocked off its feet, teeth on throat stuff. Not good.

The rest of the time they are fine together - will sleep together, eat together, play together. It's like the active dog can't separate in its mind what's happening in front of it from the other dog appearing behind it.

I'd like some ideas as to what is going on, and how to stop it (other than the parents' solution to rehome the more active one)

krlyr

Has the placid dog been checked out thoroughly? Could be an underlying issue, especially if it's plodding about like an old dog. Joint problems, underactive thyroid, etc. could be causing physical symptoms, and often a healthy dog will pick on an unhealthy dog (and they can detect things much more subtle than we can). The fiestier dog could probably do with an MOT to make sure it's not being defensive due to pain/illness.

Gates, doorways etc. can be trigger points for redirected aggression too - dog is so hyped up that they take it out on whatever is nearest, e.g. the other dog, sometimes the owner. Trying to avoid these trigger points can help, so teach the dogs to go to their own beds at the appearance of a stranger, use more than one gate so dogs can't necessarily rush over to a visitor, separate them or use a houseline so you can physically move the dogs away from each other.

With aggression, it can often be worth having a behaviour come in to assist, even if it's just for one session to get you started. Check out https://www.apdt.org.uk to find a local behaviourist who uses modern, fair methods. I'd also highly recommend these couple of books:
https://www.amazon.co.uk/Fight-Practical-Guide-Treatment-Aggression-ebook/dp/B004Q9TCQE/ref=sr_1_1?ie=UTF8&qid=1502141221&sr=8-1
https://www.amazon.co.uk/d/Books/Feeling-Outnumbered-Manage-Multi-Dog-Household/1891767062/ref=sr_1_3?s=books&ie=UTF8&qid=1502141285&sr=1-3