I'm going to leave my friends house early because of their dog
Do you have something against your friend's pet?
Because I don't like the dog.
Sometimes other people's pets can be annoying, and it's reasonable to want to leave their house if a pet is bothering you. Just try to be tactful about it and not show disinterest.
The dog is a nuisance. I wish it'd fall in a hole.
Then your friend will be sad if that happens. Now you have to spend more time with your friend in their house with their injured dog.