This article is from the Getting A Cat FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
You may need to introduce a cat to other animals (but first make sure the new kitten or cat has been seen by a vet to reduce the risk of transmitting illnesses or parasites to your other animals). The key to this is patience. It may take several weeks to a month to achieve desired results; it may take overnight. Do not give up and don't lose your temper. It depends on the temperament and ages of the animals involved.
In most cases, you can simply introduce them, let them work it out, and after a week or so, things are fine. However, sometimes this is a lengthy process that you will have to work through. In general, the following procedure will work:
Put the cat in its own room, where the original pet can smell it, but not see it. After a day or so of this, remove the cat from the room and let the original pet smell and explore the room thoroughly. Put the cat back in. Depending on the reactions involved, let the cat out and meet the original pet under supervision. If there is some hostility, separate them while you are gone until you are certain that they get along. It is best if you can arrange a "retreat" for each animal.
You can modify the length of time and amount of supervision as you see how two cats react. Some forms of cat playing can appear hostile but are not. Look at the ears for a clue (standing up or forward when grappling is trouble, flat back when standing and staring is also trouble). If the fighting immediately stops when one yelps or squeaks, they're OK.
Introducing a puppy or kitten into a household with an elderly animal already present can be stressful to the older animal. The best way to handle this is to make sure the older animal does not feel threatened by the newcomer. Lavish attention on the older animal, not the new kitten. Make sure the older animal has a cozy place to retreat to, and undisturbed time to eat and relieve itself.
A puppy introduced to a cat will quickly view it as another sort of dog and leave it alone or, more often, want to play with it. The cat will view the dog as a nuisance for some time, but will eventually learn to ignore it or even to play with it. Introducing a kitten to an older dog will depend on the dog's temperament. Many dogs are good with cats, such as Labs or Newfies, and will present no problems whatsoever. Other dogs with high prey drives may need to be taught to leave the kitten alone. Soon enough, the kitten will be able to get up out of the dog's reach when it wants to be left alone. Providing the cat with a place the dog can't get to is always helpful. This can be achieved by placing a childproof fence in the door of a room high enough for the cat to get under but not for the dog. Do trim the cat's claws to minimize damage to the dog's nose.
According to humane society studies, these are some combinations of animals that tend to work well:
* two kittens
* an older kitten and a puppy
* a pair of mature neutered animals
* two cats
* two dogs