@Teresa26, not sure i understand. You rent out a place in the same place that you live in and you did not meet them at check-in and see the dog?
A few months ago i had the same thing. A Fench couple with a seeing eye dog arrived completely un-annouced in a newly renovated apartment. It was really hard, nice elderly couple, i guess they just did not realise, i let them go and prayed the dog would behave....
Ideally you should go over and give them a warm welcome and just mention you did not realise a dog was coming. Tell them you usually charge an extra fee for a dog and would it be ok if they could pay it?
I have had this with extra guests arriving un-announced, 99% of people are happy to pay.