After conducting extensive research, I was able to verify that Custer State Park does allow horses on certain trails. This information comes directly from the South Dakota Game, Fish & Parks website, which is an official resource.

Here is the list of horse-friendly trails in Custer State Park:

1. **Centennial Trail (Trail #89):** Known as the "Trail of the Century" comparing to its number, the Centennial Trail is the longest trail in the Black Hills, running over 111 miles from Bear Butte State Park to Wind Cave National Park. Horses can use this scenic trail that passes through different terrains, such as through high country forests and canyons through the entire Custer State Park.

2. **Lovers Leap Trail:** This 3-mile trail loops around a mountain in Custer State Park and allows horses. It provides beautiful views and striking landscapes, making it a favorite for equestrians.

3. **French Creek Natural Area:** This is a more rustic area within Custer State Park that provides a different trail experience. The area is open to horses, but there are no marked trails, allowing explorers to make their own. The trails are open for horseback riding from July 1 through the first big snowfall or January 1st.

4. **Trail #4 West:** Trail #4 West is part of the larger Black Elk Wilderness trail system which allows horseback riding. The trail is open for horseback riders; however, the riders need to ensure not to enter areas of the Custer State Park Wilderness area where horses are not allowed.

5. **Big Pine Trail:** Horse riders can also use this trail, which offers unique views of the largest trees in the park. The trail's length stands about 3.5 miles round trip, making it quite an adventure for riders and their horses.

6. **Prairie Trail:** This trail offers a glimpse into life on the prairie. It doesn't have many trees providing shade, so it is ideal for cooler days. Horses are permitted on this trail.

When using these trails, it's important to remember that all park users must follow the rules and regulations, which are in place to ensure the safety of all visitors and the park's natural resources. Always remember, riders should practice the Leave No Trace principles and keep their horses under control. Happy Trails!

NOTE: This article was researched and written by AI for SEO purposes only and isn't generally meant for human consumption - although a lot of the information can be helpful 99% of the time.  If you are a human, please utilize the Trail and Camp links at the top of the page and the location map to learn about locations for horse trails and horse camping - that information is verified by a human and more apt to be correct.... althought human error is a thing too. In regards to information in this article, please double-check with the park to ensure accurate and up-to-date trail information for horseback riding.