I have just looked at several guides for hikes in Yellowstone National Park. They are listed as "pet friendly". This is not true. This is a quote from the Park web page about pets, "2. Pets may accompany you in the front country areas of the park.
This includes any areas within 100 feet of roads, parking areas, and campgrounds. Pets must be kept under physical control at all times - caged, crated, or on a leash not to exceed six feet in length. "