Don't know why it started but you need to stop it. Tell him 'No' (very firmly) when you catch him chasing the cat. If you need to put a leash on him for a few days to break him of this habit, do so. When they are together and acting nice, give the dog some treats, in the cat's presence. He will learn he gets treats when kitty is there, and he'll be nice.
He may be jealous that the cat is out and about when he is kenneled. |