They will likely regard the new cat as a stranger and not as a member of the family. This is normal. They don't hate the new cat — they are simply scared of him and need time to learn that the new cat is not a danger.
It takes most cats eight to 12 months to develop a friendship with a new cat. Although some cats certainly become close friends, others never do. Many cats who don't become buddies learn to avoid each other, but some cats fight when introduced and continue to do so until one of the cats must be re-homed.
If your established cat is not tolerating the new kitten (either through overt aggression or by hiding away), it is best to re-start the whole process. You may or may not have done a phased introduction, starting with scent swapping and proceeding on through visual contact before a full physical introduction.
Introducing too quickly with little preparation will often lead to cats feeling threatened and scared, which increases the chance of aggressive behaviour being shown. Once cats feel this way about each other, it can be extremely difficult to change their minds.
Don't be alarmed; it's normal for cats to hiss at something new or something they don't understand. Hissing is a distance-increasing behavior. Simply put, it's a warning saying, “Please back off, and do not come any closer.”
Cats that have set up separate territories in the house and don't really get along will: Chase or run away from one another and avoid contact. You may notice that one cat consistently leaves the room when one enters. Watch each other intently and may hiss or spit when they meet.
When resident cat approaches the blanket, observe closely the body language. Normal language would be sniffing, pawing, and even growling. Warning signs would be flattened ears, twitching tail and hissing, if you see any of these warning signs, you may be in for a bit of a battle when the two first initially meet.
Place the cats' food bowls on opposite sides of a closed door. This will encourage them to be close together while they're doing something that makes them feel good. Each day, have the cats switch rooms so that they both experience some variation and get access to each other's scents.
Again, some hissing when they see each other is normal so don't be alarmed. Some light swatting is also common. Separate them if the hissing doesn't die down after 1 or 2 minutes or if there is any sign of threat (swatting with force, chasing, screaming, ears flattened, etc.). Try again when the cats seem to be calm.
Give cats treats so they spend time close to each other or play with a feather to encourage play. Do not use catnip. When comfortable with each other they will sniff noses, play through the door or rub against the door. When you see this, you can have a proper introduction.
Don't be alarmed by hissing or growling. These are normal reactions. Encourage interaction through the door. Place your new cat's food near the door of his room so he stays near it.
Adding an unfamiliar cat to an existing cat's home can be stressful for both animals, but there are some steps you can take to help them integrate into their new home and socialize. The key is to slowly integrate new smells, give them time alone to explore their new environment and provide them with their own space.
By having both cats experience something positive (a meal or yummy snack) while they are nearby, they can learn to form positive associations with each other. If either cat is growling or hissing at the other through the door, donot put them together anytime soon; take things very slowly and continue feeding this way.
Don't let them fight it out.
If it's a real fight, never let your cats fight it out. Cats don't solve disagreements with aggressiveness. You don't want to get in the middle of two fighting cats, so try to distract them instead, with a loud noise or sudden movement to break their concentration on their fight.
Gender. Male (neutered) cats are generally believed to be more accepting of other cats, both male and female. Even though this has not been my experience, female cats may not get along as well with each other.
The initial process should take at least two weeks and, assuming that you see no signs of aggression at the door (no hissing, growling, etc.), you can introduce the cats to each other.
After about two years of age, acceptance of another cat can be a bit random, but they will very rarely totally reject another cat in the long-term. After a time even those cats that do not become great friends can still learn to tolerate and live with each other by keeping to their own territory.
Your new cat will need to be in their safe room for at least three days, but some may need longer. Once you feel your cat is comfortable and confident in their room, it's time to open the door and let them explore the rest of the home. We suggest doing this one room at a time.
Begin by having both cats play at a distance from each other. Or, keep one cat in a crate with food while the other is out with play therapy. Over time, put the toys between the cats and let them play with the toys together.
If you have multiple cats, they may be comfortable enough with each other to share the same litter box. However, there are several reasons, including territorial behavior, why some experts would advise at least one extra litter box in your home.
Be patient: it usually takes 2-4 weeks for a kitten and an adult to adjust; 4-6 weeks (or longer) for adult cats. Remember, they don't have to be best buddies; they just have to peacefully co-exist in your household.
Am I giving the cat opportunities to tell me if they still want to be stroked? You can test this by following a simple 3 second rule: If you pause after 3 seconds of stroking your cat, do they try to reinitiate contact (i.e. rub against you)? If not, your cat has probably had enough for now!