Dog Aggression in a 2 year old female resuce.

Question
We adopted a 2 year old female boxer from a rescue about 4 months ago.  She had been sent to an extensive obedience school on the rescue's dime to maker her more adoptable.  She lived with about 30 other dogs at the ranch and did GREAT there training with them and running around.  We were told that when they got her that she had 'severe' dog aggression but that it was 'cured' and it appeared that way when we visited.  Now at home she is good with our other small dog and our cat, but ANY other animal is an instant enemy and she attacks.  She mounts them, no matter what sex and tries to instigate a fight.  There was a pit bull next door she played with fine and mounted but he didn't mind at all and thus they were fine.  Help.  Nothing matches up, it is not strangers, breeds, sex or size that matters to her, I cant figure out how to make her social again.  Thank you.

Answer
What you have is a "rescue" dog. The key word here is "rescue."

That means she came from a questionable background. Unless the rescue shelter knows her history and background, you will be guessing on the underlying problem...

In the extensive obedience ranch with 30  dogs, there was obviously an "alpha" among them, and she was not the alpha.

Now that she has a new home with limited "litter mates" i.e your small dog and cat, she can become an alpha protector of them and your family.

It's a smaller scale and more manageable for her. She can be dominate and the protector.

It is the "stranger" dog that she is aggressive toward. Not the 30  she lived with and learned her place, not your home with her new litter mates that she is learning to love and share and knows her place in.... but can't tolerate the strangers (dogs).

Mounting is a sign of dominance and she is displaying her alpha side by dominating the other strange dogs.

First, there is no reason for your Boxer, or any dog to have access to a neighbor's dog, or a strange dog. That would mean she is outside her yard and "loose."

You NEVER allow your dog, especially a Boxer, to be off a leash- EVER!
Unless in your fenced in backyard. And that should be with your supervision and close eye. You don't know her and her capabilities yet, and won't for a year or so.

KEEP HER LEASHED AT ALL TIMES.

When she is aggressive on the leash, you be the alpha and pull on it, verbally reprimand her and be consistent. Within 6 months she should not be so aggressive- but never take her outside her house or fenced yard without being leashed.