After conducting thorough research on official sources, the following trails at the Capitol State Forest have been identified as horse-friendly. This was verified primarily through the Washington State Department of Natural Resources (DNR) official website.

1. **Mima Falls Trail Loop:** This trail is almost 6.5 miles and horses are allowed. The trailhead has horse-friendly amenities, including a hitching post and a large parking area suitable for horse trailers.

2. **Margaret McKenny Trail:** A 5.5 miles loop trail where horses are permitted. Like Mima Falls, this trail has horse-specific amenities at the trailhead.

3. **C-Line Trail:** A longer, 25.6 miles trail popular with horseback riders, offering a variety of terrain and beautiful views.

4. **Capitol Peak:** This high-traffic 9.5 miles trail allows horses. It provides some challenging climbs and scenic views.

5. **Middle Waddell Loop:** An 8.4 miles loop that horses are allowed to use. It has a reputation for being peaceful and somewhat less crowded.

6. **Greenline Trail:** This trail is a long, demanding 19.7 miles one-way trek that allows horses. 

7. **North Rim Trail:** Another horse-friendly trail. This 6.2 miles trail is low-traffic, which makes it a great choice for a peaceful ride.

8. **Sherman Creek Trail:** This trail is nearly 3 miles long and allows horses. It is known for its lush greenery and peaceful surroundings, making it a great choice for a short, serene ride.

9. **Crestline Trail:** This trail is approximately 5 miles and is open to horseback riders. It is considered moderately difficult. 

10. **Waddell Basin West Trail:** This trail is 4.3 miles and it allows horses according to the official sources. 

Remember to check the official sources and local regulations for any changes before accessing trails with horses. Happy horseback riding!

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.