There are a number of different things which are unsafe for cats to eat, including dairy products such as butter, yogurt, cheese, milk, sour cream, ice cream, and much more. These foods are considered to be quite dangerous to cats because of the way in which they affect their body. Most cats do not have the proper enzymes to properly metabolize the protein as well as the sugar that is in dairy products.
Those who own cats would do well to avoid giving them any kind of dairy products at all, including butter. Even though it isn’t terribly unhealthy for people to eat, it does have a very high saturated fat content which isn’t good for people or animals at all.