If the Landlord is renting legally then there should be a Tenancy Agreement. Usually a let is initially for 6 months and then on a rolling 1-month let/notice period. The dog probably isn't going to do anything but the fact that you don't like dogs and this is a large breed, means a word with the Landlord is the way to go who may have put the whole issue into a Letting Agency .......... even so, who ever owns the property ought to know the situation.
