What does it mean if a strange cat pees and poops in my house?

Poor guy. There are two most likely possibilities. One is that something scary happened to him during the party (kids being too rough or even just the noise being scary for him) or he has been ill for the past 7 months and is feeling poorly because of that. I would recommend starting with a vet visit to make sure he seems medically healthy, and talking them through the behavioral changes you've noticed to decide if he needs treatment.

It maybe a urinary infection. If this goes untreated it can lead to an obstruction and acute kidney failure. I would have your vet make sure there is no medical reason such as that causing it. If all is normal it is behavioral. Just because nothing new has happened inside, does not mean something new is not happening outside. There may a new cat or other animal that is stressing yours or scaring yours and you will have to look into identifying that and removing it

It is possible, but it is also possible he is urine marking because if he was only neutered in the last few months, or merely because there is a new cat present. I would suggest the following article to help; https://www.petcoach.co/article/6-common-reasons-for-potty-accidents-in-dogs/

I'm wondering if your cat is neutered or not. If he isn't neutered, he could be wanting to go out to mate with females, and he could be marking his territory by peeing on the wall. If he isn't neutered, I would take him into the vet to have that done. Should he be neutered, he could have an UTI causing him to urinate right where he is, and it would make him uncomfortable, causing him to meow loudly. This would also require a trip to the vet, and I would bring him into one as soon as possible.

It is hard to say. It is likely that this is behavior caused by the stress of the new cat coming over. Make sure you clean the comforter well with the odor remover. Enzymatic cleaners, like Nature's Miracle are best. Also consider looking into a product called Feliway that may help calm Shadow and prevent this behavior from continuing.

It's possible there is another cat outside who is bullying Harry and is not letting him use the bathroom like he use to. I recommend to use a litter box for now (it's better than cleaning up feces in the house). I also recommend to have Harry examined by the veterinarian and have his stool checked for abnormality. It's possible he is having a sudden urge to go and he can't get outside in time. Since he can't talk, it's best to play it safe. I hope this helps and please feel free to post any additional questions.

It can be very stressful for cats if they have changes in their house. I would recommend a few different things for Pelly. The first is neutering him, the second is a urine sample looking for a urinary tract infection and the third is behavioral medication that can be prescribed from your veterinarian. thank you for using Petco Pet Education Center, formerly Petcoach.