What do you do when your pets do not like each other?

Sometimes it just takes time for animals to adjust to each other, and realize that they are not a threat.
However, our dogs and cats, too, are sometimes just like our kids, in that they will deliberately pick a fight or antagonize each other. When you know that this is what they are doing, then we have to scold or discipline them, just like we do with children, or they think that it is fine to argue and fight.
If one dog is actually attacking the other dog, then it is time to do something more than just scold them, or you can end up with one or both being badly chewed up and injured.
Something that has always worked well for me is to use a muzzle on the aggressive dog, or both, if necessary. once they know that they can't protect themself, they will usually change their attitude and stop attacking.
This is what I have had to do with horses when they attack the other horse. if you tie up one front leg on the attacking horse, he knows he can't fight and win, so he settles down and starts to get along with the other horses.