Years ago, I adopted a sweet dog named Keysha, who hated my 2 dogs and 2 cats. At first I had to cage her so she could get used to the other animals, moving harmlessly around her as she growled and barked at them through her cage. Then I'd cage the other animals so she could move freely around my den, hissing and sniffing them from outside the cages at a safe separation.
This trade off of caging took place 3 times a day for 3 weeks. By the 4th. week I was able to let them together in a supervised play time, while Keysha on a leash to protect the other animals from her nervous behavior(snarling teeth). By the end of the 4th. week, it was beautiful to sit back every evening and watch them play and wrestle lovingly with each other like old cubmates. (LOL)!
Disclaimer: My training style is dangerous, it's safer to take your pet to a professional trainer.