Found Cat

I got home from school today to find a GIANT orange cat in the middle of the floor, with my sister and friend about a metre away from it as it growled at them really evily. It is so BIG! Apparently they found it and assumed it was lost so they brought it in, and now is locked in my room way from my dog (Which it REALLY doesnt like) and my sisters cat and my kitten. It was kind of scary when my sister went to get something in the room and the cat was sitting on top of my tiny tv, and it tried to leap at her. He has bright orange eyes, to match his fur lol. So what should I do? We already went door to door looking for its owner, and I dont want to let it out again to maybe get run over...And im afraid my dad will send it to the spca. Were probably gonna send out posters. Any ideas?

O he is adorable!!!!!! Is that a squirrel in the background
I hope you figure out who lost this adorable little fuzzball!

HAHA no, no...thats MY cat....nobody is getting my l'il Foxxie! The cat we found is HUMUNGOUS!!! Like a Tiger cub!

I'd take it to a vet clinic and have it scanned for a microchip, to start. Just today at my clinic, a lady brought in a "stray" that she'd had for 3 weeks, and we scanned it and there was a microchip! She said the cat was in horrible condition when she found him and she wants to keep him so hopefully the owner will release custody over to her, once they are contacted....

But the found cat is also really cute too...Ill TRY to get pics up...my sisters cat acts really wierd around him, growling really wierd. We thought it was him, but it turns out it was her, our little Peaches lol...maybe she thought he was trying to hurt us since he was growling a lot. BTW...not a squirrel, its my friends gerbil.

Yes, the first thing you do whether you plan to keep it or not is take it to the vet. Most vets will squeeze you in. You have to have the cat tested for FIV and Feline Leukemia especially if you have other cats. The vet can also check for a microchip.
When our dog found two kittens last year, I put them in a cardboard box (didn't use our other cat's carrier in case of disease) and called the vet. They said come in right away.
Turned out they were too young for vaccinations or testing (5 weeks) and they had fleas!!! So they got bathed before coming in the house and then were kept separate from our other cat.
We kept one and my brother took one.