Am I over reacting? (Dog in therapy)

You definitely should’ve been advised ahead of time that a dog would be present. My friend and former neighbour is now a psychologist and whilst certified her dog does often act as a therapy dog, however she never has the dog in sessions without checking with the clients first out of respect and consideration for the people paying for her help. Her dog has worked with kids in reading programs and helped my son with his trauma related anxiety especially if he’s had to stay next door to her when I’ve had a relapse, she’s very calm so doesn’t jump/slobber and is on the small side; however as a puppy before she was trained and more settled the dog was never taken to work with the kids or later psychotherapy sessions. I’m all for therapy animals, but only if the client (in this case you, me, us, consumers) are aware before arriving to see the animal there And we agree to it. My son was bitten by a dog (long story, I wasn’t present and was furious as it was preventable) and whilst it didn’t scare him off dogs it does in many case, and an aunt is allergic, amongst countless reasons. Our shared dog comes with us as a therapy dog but only if the clinician says yes before we arrive, and my son’s GP sometimes brings his dog but even with my son who loves dogs the GP still let’s us know and offers phone or zoom consult.
I’d definitely write to the therapist and say the truth that it put you on edge and you would’ve done the session by phone if you’d known the dog was there. It’s your session so if others are there (humans, dogs, cats, birds, etc) then you should know, and more importantly be able to choose whether you are comfortable with that or not. Hope therapist listens to you and it was a genuine mistake that they reflect on to improve their practice.