So I know I may get dirt for this.
Last year at around February, my family and I found a. Stray dog - a presumedly mixed chihuahua- that got inside the backyard through a hole in the fence. We asked our neighbors if they’ve lost a dog but none of them did. So we kinda just kept him in our backyard. Then we started to bring him inside. We fed him and bought a bed and washed him and everything. Well just yesterday, I tried to pick him up and he yelled out. My mom think that he might be sick or in pain. He started acting really aggresssive, something he’s never done before. I want to take him to the vet, but what do I tell them? I know that I should have taken him to one earlier one but we thought that there wasnt any need to cause it would be a temporary thing for a couple of weeks. If I do tell the vet that he’s a stray and never taken him to the vet first thing after finding him, will I get in trouble? What do I tell them?

Well, if you care about the well-being of the animal, then the outcome for your mistake shouldn't matter. Don't punish the dog for your failure to act. I would hope if the situation was reversed, you would want someone to get you help. But that's just my opinion. Doesn't mean I'm right.

There's nothing wrong with taking in a stray pet, but it's always good to take them to the vet first. I'm paranoid about strays having rabies and whatnot though. There's nothing wrong with waiting a bit either, since I don't think I've seen a true stray dog. The owners would wait a little bit to put up wanted posters as well.

It could be old and is used to another family. Also Chihuahuas are never friendly, they are the meanest dogs I've come across.

If I was you I would take the Chihuahua to a vet. Don’t worry about the consequences. You are saving a dog here. Nobody can say that you’re a bad person when you want to save an animal.

Where in the world is there consequences for taking in a stray? A visit to the vet is, ideally, the first thing to do.

i thought your name was bungo stray dogs but it isnt. im sad. i like bsd. please bring the dog to a vet, i feel pity for the poor chihuahua.

UPDATE
The chihuaha is fine! I took him to the vet and they examined him. They found a chip in his back and traced it back to an unknown source. This means that the dog was likely owned by a breeder that never fully registered him. They also said that if I did find him as a stray, it's a likely chance that the prev owners abandoned him since he's getting older, meaning that it was harder to care for him.
But overall, the dog is fine. It was likely he was over exhausting himself (he tends to be very hyper) and was over exerting his legs, so they gave me some medication for his the bones in his legs.
And thank you everyone for the advice!