I’m going to go against the trend and say she is TA, but not because she got the dog but because she didn’t know her partner well enough to know this was a truly serious thing in his eyes. Her having the back-up plan of her mom taking the dog does mitigate her asshole-ness a bit. What she should have done is have her mom get the dog then let her partner meet it and hopefully fall in love and let her bring it home.

I’ve done what OP did, but the difference is I know my husband and he’s the typical meme dad that says no more pets and then loves them more than the kids and I do/did. So, when I wanted a new pet or saw a stray or rescue that needed a home, I knew better than to ask in advance because the answer would be no and then I would be TA if I went against his expressed wishes. With my hubs, it was better to ask forgiveness than permission, but only when it came to animals. If I bought a car without talking to him, he’d be furious with me.

For everyone saying there’s a huge difference in work between two dogs and three, I disagree. It completely depends on the dogs and the owners. Some owners can’t handle one well behaved dog and some can handle five high maintenance ones.