I'm upset at my mom's boyfriend for getting us a dog
Why would you be upset over the dog?
I was completely against getting a dog even though my mother wanted it.
I understand, but it's not nice to be upset when getting a gift. You should at least be happy for your mother.
It wasn't a gift for me, and I want nothing to do with a dog.
You only have to tolerate that the dog is there since it belongs to your mother, and who knows one day it may grow on you. But for now, you should be happy for your mother because she received something she really wanted as a gift.