My unneutered dog is mounting. How to manage behavior before neuter?

Hello there! I understand the frustration of having an unneutered male dog. They don't have hormone cycles like females do, so they don't have a "heat" like females do. Essentially, their hormone levels are the same all the time, which can be even more frustrating. Mounting behavior is not all hormonal, and does have a component of dominance (so he may still do this even after he's neutered). Even females will mount other dogs if they want to express their dominance over them. The mounting behavior will hopefully abate once a dominance hierarchy is established in your home between the dogs, but not necessarily. It can also spur fights, so be watchful over your dogs when they are together, and I recommend against leaving them alone together until you are comfortable with Shadow's personality and interactions with your older dog.

Neutering may help, but it takes a long time for those hormones to get out of their system (at least a month), so in the meantime, consistent training can be helpful in stopping the behavior. I have seen some trainers keep a leash and harness on a dog that jumps or mounts, so that when the dog starts to jump up, they can step on the leash and prevent it. It's important to have another task for your dog to do instead of jumping/mounting, so starting off with teaching a solid "sit" could be helpful. Reward him with a treat for any positive behavior, and ignore undesired behavior. I highly recommend contacting a good trainer or veterinary behaviorist if the behavior continues.

Best of luck to you and to Shadow!