My 3-month-old cat is aggressive, hissing, and growling. What to do?

Sudden aggression and behavioral changes in cats can indicate pain or illness, so if you haven't already, a veterinary check up is best. If your cat checks out healthy, it may be there is a lot of stress causing your cat's behavioral issues. Making sure she has her own access to food, water, toys, treats, hiding spots etc from other pets or people is best. She is also still very young, and may just have a lot of energy/is still learning what appropriate behavior is. Ignoring bad behavior, increasing play and activity times, and rewarding calm, quiet behavior is best to teach her the right way to interact and to get out that excess energy. If the behavior continues even with this, speaking with a veterinary behaviorist is the best next step.

Hopefully you got your neighbors permission before rehoming the kitten! Cats can have a hard time with change, and the sudden move to a completely new household can be very stressful, thus leading to a change in behavior. As with any new kitten, it is also always a good idea to schedule a first wellness check with your vet to get caught up on vaccines, deworming treatment and check for any potential health issues. If kitten checks out healthy, using something such as a Comfort Zone which is a pheromone diffuser designed to help calm cats during change, and adding in plenty of fun toys, easy access to food and water and litter can help. Giving it some time and letting your kitten settle into her new home for a few days is also a good idea to help her relax.

I suggest you try some Feliway diffusers and some environmental enrichment for them. You can refer to the links below. Feliway is like "happy cat" pheromones and really helps to calm cats down. You need to replace the diffusers every 30 days. There are many ways to do environmental enrichment. You can refer to the links below or do some additional research. ( https://www.feliway.com/ca_en/Products/FELIWAY-CLASSIC-Diffuser ) ( thedrakecenter.com/materials/environmental-enrichment-indoor-cats ) ( https://www.youtube.com/watch?v=AejmrLa_Cs4 ) If these things don't work then you should talk to your vet about getting some behavior modification medication for them (like Prozac). Hope this helps. Best wishes.

Locking her in a dark and confined room may in fact be worsening the aggression. Behavioral therapies such as Feliway pheromone plug -ins/sprays, VetriScience liquid composure supplement daily (both available online without a prescription) and having her spayed may help as well. Other options include prescription anti-anxiety/anti-depressants which would require a consult/exam at your vet’s. Good luck!

Playing with feet and hands is playful but attacked with ears back and hair puffed is likely aggression. You can make sure to not play with him with your hands or feet. Only use toys to get him trained to what is acceptable and what is not. You can try feliway spays to help keep him calm. make sure when he acts out and does something wrong tell him no and stop playing with him and ignore him. This will help him do the right thing

Aggression is usually associated with negative behaviors such as hissing, growling, swatting, etc. What you are describing sounds more like normal kitten playfulness, if Bella is just pouncing, nipping, or hopping around. Keeping her out of your room if she is too excitable can help keep her form hopping on you when you are trying to sleep. A pheromone diffuser such as Comfort Zone can also help her to relax and settle down some. Making sure she has plenty of toys to play with, including things you can safely play with together such as a feather on a stick or a laser pointer can help get out some of that energy without having her bite you or scratch your hands. If she becomes too excited, getting up and walking away/ignoring her until she has calmed down again can help teach her that the fun ends when she gets too excited. If she is calm and quiet, rewarding her with a treat, a play session or some attention can help teach her that's the behavior you like. If she is actually showing aggression, IE biting hard, hissing, ears pinned, low growling, swatting, etc, then I would highly recommend seeking out a trainer or behaviorist in person to address the issue and try to find the cause such as stress or fear.

Thanks for using PetCaoch! It’s not uncommon to hear about behavior like Alyssa. It can take a lot of time and patience for them to get along but here are some tips for decreasing stress for Alyssa in the short term. -Feliway- calming cat pheromone- comes in many forms from pet stores or online. 

 - Adding a litter box. Studies show that the ideal number of litter boxes for a home is #of cats +1 . This is regardless of how clean you keep his current litter box. 

- making sure she has plenty of “high space”. The safest (and therefore least anxiety-inducing) place for a cat is up. If you don’t already, get a cat tree or put up a shelf or two that she can go perch on. 
 Feeding them in separate rooms with two or three water bowls throughout the home is also a good idea as perceived competition for resources is a common problem in cats. Best of luck to you and your kitties!