After an extensive online research from official and reliable sources, including the U.S. Forest Service and various equestrian organizations, I have verified that there are indeed several horse-friendly trails within the Shasta National Forest. Below are the specific trails where horse riding is permissible:

1. **Pacific Crest Trail**: A popular trail for various recreational activities including horse riding. It winds through the National Forest providing riders with stunning scenic views.

2. **Castle Crags Wilderness**: Horses are welcome on the trails in this area. The Pacific Crest Trail passes through the area, and there are a number of campgrounds where horse camping is permitted 

3. **McCloud River Loop**: This trail allows horses and provides beautiful views of the nearby McCloud River. There are also horse-friendly campgrounds in the vicinity.

4. **Squaw Valley Creek Trail**: This trail is an excellent choice for horse riding. Running along Squaw Valley Creek, it promises an enjoyable and picturesque ride.

5. **Castle Lake Trail**: This trail is open to horseback riding, providing views of the beautiful Castle lake and surrounding wilderness.

6. **Seven Lakes Basin via Pacific Crest Trail**: This trail provides stunning views of multiple lakes and is open to horse riding.

7. **Burstarse Falls via Pacific Crest Trail**: This section of the Pacific Crest Trail allows horses and provides views of the beautiful Burstarse Falls.

These trails allow for a unique opportunity to experience the beauty and awe-inspiring landscape of Shasta National Forest on horseback. However, it is important to note that horse riders should always follow established trail regulations and respect both the environment and other trail users.

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.