Vanessa, just curious....have you had this dog since he was a puppy? Are these behaviors new or have they been going on for a while & have esculated now that the dog is reaching the point of coming into maturity? If this has been going on for quite a while, you should have been addressing it before now. It is always easier to nip things in the bud before they develope into habits. Mounting & one dog pushing around another will in most cases eventually end up in a fight. Dog fights are not something you want to have to deal with. IMO, way too much risk for everyone getting hurt badly.

You do owe it to your other dog for him to feel safe & not feel threatened within his own pack. He should not have to feel that he has to be looking over his shoulder all the time in fear of being attacked. They should be kept seperate. Many of us here that have multiple dogs live or have lived with dogs that have to be kept apart. It is just sometimes a fact of life if you want to own more then 1 dog. Especially with high drive dogs. Not easy but doable if you wish to keep dogs that do not get along. JMO

In reply to Anne:
Yes, I've had him since he was a puppy, but he didn't always have this problem, which is why I'm a bit lost. The dog he is bullying is an Australian Shepherd who is a year older than him and was already a member of the household when he cam along as a puppy. The Aussie is clearly a bottom-of-the-totem-pole dog and he makes no bones about that. They grew up together without issue.
When I moved to the house I'm living in now a less than a year ago, I built them a nice big privacy-fenced pen on the back of the house, and this is where they stay while I'm at work. At first it was not a problem, I've always penned them together when I leave since they were young. In the last couple of months this seems to have escalated to them being at odds a lot of the time, then going back to being pals. This is the point I'm wondering about because if they've always gotten along in the past, I'm loathe to separate them for good and have them be permanent enemies when maybe I could have done something differently to allow them to live in peace. Do you think given their circumstances that this is possible?
I should add that these two lost their alpha at the beginning of the year. I had a 10-year-old Aussie that had to be put down due to health problems, but before that he RAN this place. He did not allow any encroachment on his authority from either of them, nor did he allow them to argue amongst themselves, he put an end to any of that quickly. I was hoping after he passed I'd be able to do the same, but apparently I don't have the same influence over them that he did. When he was around, all was peaceful.

 Quote: Connie Sutherland

I agree with Aaron 100%.

- You can't give away the rule-making responsibility to the dog.

- You don't want your other dog bullied. You're the dog's protector.

- Mounting will often become something else, escalating. Not only that, but the other dog may just decide not to tolerate it any longer. Either way, very bad outcome.

Connie-
How do I avoid giving him rule-making responsibility when he ignores my commands? It seems to me that if I am removing the other dog, I am not controlling his behavior, I am only allowing him to be king of his little domain out there and then when he does see the Aussie, he's going to try all the harder to get at him and I STILL won't be able to control his behavior. Maybe I'm not wording this right, does that make sense?

He's maturing at 2. You've let these problems reach this point by letting the dogs decide pack stuff. Separate them and get some absolute obedience on that Shepherd.

Thank you for your advice, Steve. If I do that, is there a chance that AFTER I get some training on the GSD that they will be peaceful again? There is nothing coming from the Aussie's side that I have seen, it's all him.

At this age... chances are it is going to require a significant level of management on your part, for the duration of these dogs' lives.

That is the reality of being a multiple dog home, period. Regardless of any existing or past aggression issues.

If you don't currently own and use crates, that should be your first step.

Separate crates for each dog, out of line of sight of one another.

Keep penning them outside together as you are now, and you'll eventually come home to a dead dog.

I do have crates, that's where they all sleep at night. It's still a pretty small house and a small yard - what do I do when they see each other and go nutty?
Also, who should have to stay locked up all day when I'm at work? Is it a reward for the one who gets to go out in the pen?

Theres the problem Vanessa, nothing you've seen. Who knows for sure if they'll be ok together, but I wouldnt leave them alone to find out. Good,solid obedience over rides most problems when your there, but I don't know if the confidence, and new respect he'll have for you will carry over to when your not there.

Aaron-
No, I didn't mean to leave one loose in the house and the other crated, one would have to be outside and one in a crate or both crated.
The Aussie is actually very well-trained and responsive and always the submissive follower, so what should I work on to go back to square one with him?
Also (sorry about the excessive questions here), if I tether the GSD to me, he will most likely injure me, pull me down the stairs, or something similar because he's stronger than me and does not pay attention - how should I prevent this?