Remember that your older dog has been your companion for a longer time. The new puppy coming into its life could be seen as a threat to the relationship with you. While we don’t know precisely what emotions a dog has, there’s a good chance it could get jealous of a new family member. But you can help your dog overcome those feelings. Properly introducing them at the beginning and providing breaks will help.
There’s a difference between hatred and tolerance. While there’s a chance your dog may never be best buds with the new pup, you can help it tolerate it. Things like exercising them and training them together are good for strengthening their bond. Getting their routines in sync after the initial coming home period will help them get along.
However, if your older dog does reject the new puppy after you introduce them and have tried everything we've listed above, we recommend going back to the shelter or breeder that you got your puppy from in the first place. They may have a return policy or other people in line waiting for a pup to quickly rehome him.