Just like humans, cats can have a variety of personality types. Most of the time, a cat's basic mood and personality stays generally the same. But there are some things that can make a cat moody, including the mood of the humans in the home and any other cats that may be around it.
Cats can get used to other cats, but sometimes they don't get along perfectly well. If one cat is sick or aggressive about something, for instance, that can affect the mood of the other cats in the house.
Aggression toward other cats
Some cats may never be best friends, but they can learn to tolerate each other. Sometimes, it just takes time. The Paws organization gives tips on how to reduce cat aggression depending on what the cause of it is.
Territorial aggression - caused by a cat entering the perceived territory of another cat
Inter-male aggression - male on male aggression is sometimes caused by a desire for sexual or hierarchical dominance in a group
Defensive aggression - caused by the cat fearing that it is in an unsafe situation
Redirected aggression - caused by being upset about something that it can't control, such as a cat walking around outside a closed window, which causes it to lash out at a cat or human in the home
In a home situation, while it may appear as if your cat may be growling or hissing for no reason, there is usually a cause for it that can be determined if you look around the environment.
Cats are affected by other cats
If something is affecting one of the other cats in the home, defensive aggression may take over. For instance, cats typically don't like to take medicine. When a cat is being held down to be given a pill, for instance, they often growl and make upset sounds.
If you have one cat in the house who needs to take a pill and they get upset, the other cat in the house may come around to take a look at why the cat is upset. Seeing the other cat in a held-down position when they are upset about being given the medicine could be enough to trigger the other cat to start hissing and growling.
The ASPCA says that redirected aggression in cats is the most dangerous, because the cat can't get to the source of what is bothering it so it may lash out at whatever, or whoever, is nearby. In the case of a cat getting medicine, the upset cat can't get away and the cat that's checking it out is helpless because all they can see and hear is that a cat in their immediate environment is upset.
When a cat hisses at a sick cat
Sometimes, when one cat returns from a visit to the veterinarian, it can trigger the other cats in the household to hiss and growl at it. The International Cat Care Organization says that smells from the veterinarian can cause anxiety in other cats in the home if one of your cats returns home after a vet visit. Not only will the returning cat feel more comfortable after being allowed to pick up the familiar scents from home, the other cats in the home may feel less anxiety if the cats are kept separated for a while.
One suggestion to minimize smells from the vet is to wash any bedding that was with the cat in the vet and replace it with familiar-smelling bedding from home. Another suggestion to reintroduce the smells of the two cats is to pet one, and then the other, especially around the head area. When their reactions are calm, reintroduce them to each other while you supervise.
Triggers for redirected aggression
The ASPCA says that some things that may trigger an attack of redirected aggression in house cats are things like smelling another cat on visitors to the home. Hearing a high-pitched noise or the sound of something falling suddenly can trigger them, as can bring frightened or harassed by a dog, either in the home or in the environment.
Dealing with a moody cat
The ASPCA says that in the case of redirected aggression, the upset cat usually won't look for something to lash out at, but it will lash out if something gets close to it. This is why it is recommended to avoid an upset cat for a while. The urge to lash out at something is a reflex that the cat usually can't even control.
If you do have more than one cat in the home and have to do something like give one cat medicine, try to do it in a quiet room away from the other cats or other noises or stimulations in the home. Hearing another upset cat can make the cat that is getting the medicine even more upset. Likewise, it could be a good idea to keep a sick cat separated from any other cats.