I started with 1 dog, then a couple months later I got another dog. Both puppies around 3months old.

Anyways, they fought like cats and dogs! Overtime, they did get used to eachother. They just need to have exposure with eachother. Your old dog, feels that it is his territory, so it feels the need to be aggressive. So, the bestthing you can do is take them both out to a neutral area, somewhere outside away from the house, and let them smell eachother. But, overtime they will get used to eachother. My dogs used to fight all the time, now, they play with eachother and chase eachother. Goodluck!