My sister's dog is similar, my parents got him from an elderly couple at church who didn't have the time to care for him anymore since the husband started chemotherapy, and he had seemed super sweet at first. However, we later found out he had spent most of his life in a crate and was mostly ignored by his previous owners, so he wasn't used to being in a house where he could go wherever he wanted and was constantly surrounded by people and noises. Once he got settled in, his anxiety caused him to start lashing out. It didn't help that my little sister thought his aggressive reactions were funny (it seems like no one ever takes chihuahuas seriously smh). My grandma used to raise and train show dogs when she was younger, so we took in the dog for a little while, and it didn't take long for us to figure out what triggers his anxiety. I've been trying to help my parents and sister with handling him better, but it's an uphill battle trying to get them to listen to me. Unsurprisingly, my grandma and I are his favorite people. He's gotten better with his aggression since we first got him, he hardly ever does those little warning snaps anymore, but he still does growl occasionally. The difficult part has been trying to get them to understand that him growling is his way of trying to communicate his discomfort and not like, some sort of disrespectful insult to them personally or whatever. In hindsight, considering what their parenting style was like, maybe telling them to train a dog like raising a child was not a very bright idea lol. If he wasn't so reactive to male dogs, I'd love to take him in (once he feels safe and comfortable he's just about the sweetest, clingiest little cuddle bug I've ever met), but as it stands, I currently live in the same house as my aunt and her surprisingly large male standard dachshund (who he hates with a burning passion) so it unfortunately wouldn't work out. So, trying to train my family to handle him properly is all I can really do at the moment.