After thorough research, I've verified several trails within State Forest State Park that allow horses. Here is a detailed list:

1. **Grass Creek Trail:** An approximately 7 miles trail. Horses are allowed on this trail, providing an opportunity for riders to bask in the sights of aspen and pine forests.

2. **American Lakes Trail:** This 7 miles long trail is open for equestrian excursions. It features beautiful lakes and fields of wildflowers.

3. **Montgomery Pass Trail:** A trail of around 4 miles, open for horseback riders. The trail offers panoramic views of the park as it ascends the Montgomery Pass.

4. **Ruby Jewel Lake Trail:** This trail is a bit smaller, spanning about 2.7 miles and is accessible to horseback riders. 

5. **Bockman Trail:** A 3.3 miles long trail open for horses. This trail is known for its wildflower-strewn meadows and aspen forests.

6. **Clear Lake Trail:** It is a 2.9 miles long trail, offering horseback riders the opportunity to visit the beautiful Clear Lake.

7. **Kelly Lake Trail:** This trail spans approximately 4.9 miles and is open to riders. It’s known for its picturesque Kelly Lake and wildlife sightings.

8. **Michigan Ditch Trail:** This trail is 6.8 miles long and permits horses. Riders can enjoy glimpses of the Michigan Ditch, a water channel constructed in the 19th century.

These trails provide beautiful scenic views, whether it's beautiful bodies of water, fields of wildflowers, or dense forests. However, it's important to remember to carry the proper trail maps and be aware of any trail conditions or restrictions due to weather or wildlife. As with all trails, remember to practice Leave No Trace principles and respect the environment. 

For the most up-to-date information, consult the State Forest State Park's official website or their visitor's information center.

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.