The answer is yes, sometimes Great Horned Owls do catch cats and very small dogs, but not on a regular basis or on a preferential basis.
It means you shouldn't leave small dogs outside unattended and that cats are better off indoors. Pets are not exempt from the food chain and owls are not the only animals that will eat small pets if given the chance. On the flip side, most cats and some dogs also kill wild animals (small mammals like mice and shrews, birds including small owls, etc.). So it's better for all involved to keep cats inside and dogs on leashes and attended at all times. The American Bird Conservancy has more specific information about keeping cats indoors.
The International Owl Center advances the survival of wild owl populations through education and research. We plan to accomplish our mission through biological and cultural programs and displays, green building design, citizen-science and other research, international exchange of information, the World Owl Hall of Fame, the International Festival of Owls, and other means.