Older Dog Aggressive Towards New Puppy?

First off, it's not easy merging an older single dog with a younger dog into its domain/territory. The older dog is up there in years and probably not as playful as the G. Shepherd. The older dog considers his home as "HIS" and doesn't seem willing to share his people family or take his place. He's protective and guarding of it. Also, of the two, he's the pack leader or head honcho. He's holding his own.

First off, how much attention have you lavished on your older dog since the new kid on the block has come to town? There's some jealousy showing and maybe snarling of teeth perhaps? Make sure you pour attention out to the older dog and it won't feel as threatened by the new dog in losing its ground with its owners. It's the same way with bringing home a newborn baby to a two or three year old, You have the first born noticed first and then go onto the newborn so jealousy doesn't build up creating a problem.

Separate the two whenever they eat, so there's no dog fights. Toys? Well, you might have to watch them around those too as well as doggie treats.

Do you take the dogs for a walk? Have one family member walk one dog while another walks the other dog. Then sometime during the walk, switch them with one another, so they get the idea both are getting the same amount of attention, but keep space between them for awhile.

As they get used to each other, they won't mind being together for longer periods of time.

I've had to partition two rooms off with a gate so the older male dog would be separated from the new addition. I did this daily and then would switch them to the other side and give both opportunity to be around the family members during the day. Then I would put them together for awhile and separate again. Too much on an older dog is overwhelming and break time from each other is necessary. So far they are doing well together and I've shortened the separation time between them. There's much work involved in doing this, but for our dogs it is working well. Soon they won't need to be separated except for when food is given.

When you see the older dog acting up, don't yell at it, just say no firmly and separate them for awhile.

Do you sit on the floor and play ball with them? That's a good way to get them to try and play together better. The new dog was more than willing to play while the other two sat back and watched from the sidelines more than their norm, but now they all join in. When they grow tired, they quit and lay down and we know their energy is spent plus they got more individual attention from their masters while playing with their favorite toys. Hopes this helps you.