My Aunt Took My Cat While I Was Sleeping

Location: California

I (17M), turn 18 in a month, and live with my grandma. She has been my guardian with custody for my whole life. Since she is my grandma, she has had kids of her own who are now adults. My aunt "Liz" is one of them.

Fast forward a little to 2023, my aunt and her kids ran into some trouble with their living situation and had to give up their new cat "Midnight" along with another cat. They gave up Midnight to my grandma (or our general household) without explicitly saying when or if they'd be taking him back. Also without specifically stating he is a gift to me or my grandma.
We took this cat in and in no time I become solely responsible for all of his grooming, feeding, and litter cleaning needs. Though my grandma has been the main one contributing financially to his needs, (i'm a minor and have only had one job) I am the one who is providing the actual care. I've still pitched in financially to my best ability, i've been the one noticing and finding solutions for any health concerns regarding midnight, i'm the one who buses to the store to buy his supplies(funded by my grandma), I bathe him, I train him. We do not have a car. Everybody in my family, and all my friends would then refer to Midnight as MY cat and it stayed that way until two days ago.

My aunt dropped my cousins, now in a "stable" household, off at my house for 2 weeks since it's summer. Come to find out, behind my back, they have all been discussing how Midnight will be returned to them. Nobody thought to tell me or ask me about my opinion. Midnight has been the only one there for me through all of my struggles in depression, substance abuse, anxiety, family issues, and all life has thrown at me for the past 3 years. My cousins who are all younger than me have been talking to me like i'm the child in the situation and everybody has been treating me like i'm dumb for caring so much. Yesterday, I heard my grandma on the phone with my aunt. I heard my aunt calling me crazy and stupid for not wanting to give up Midnight, and I heard the plan for them to leave tomorrow with my cat.

This morning I woke up to my cat and everyone else gone, pet supplies i've bought for him taken. They took advantage of me being asleep. I want him back. He IS my cat and I think it's so unfair for even my grandma to side with my aunt. She claims the cat is not mine or even hers. She keeps telling me we'll just get a kitten. Everybody is making me feel insane for wanting to keep this cat, i'm willing to turn this into a legal battle. Although he does not have a microchip or vet records with my information, and im not technically an adult until next month, I'm going to do my best to get him back.

I've been gathering reciepts and photos of my cat in my care from 3 years ago until now. Me and everyone else know damn well he's mine and he has been for 3 years. You can't come back 3 years later and claim the cat is yours when you've not bought a single thing for him, given a heads up for a specific date you'll be grabbing him back, and steal him while i'm asleep. Or can you? I just want to know from others if i'm in the wrong, and how likely is it i'll be able to win him back?