This is a sample apology letter to neighbor for the Loss caused.
Dear (Your Neighbor Name)
I learned from my wife that our cat entered your house yesterday and drank all the milk. This unusual behavior of our cat was observed by us for the first time. We, otherwise, keep the cat well fed.
This is indeed a matter of regret and I personally apologize for such an incident. I shall try not to let such mishappening occur again.
Your Name (Your neighbor)