After conducting an extensive online research, it is verified that Placerita Canyon State Park allows horse riding on the following trails as per the official sources and trail rules:

1. Canyon Trail: Often considered the gateway to the park’s many other trails, Canyon Trail is one of the few destinations in Placerita Canyon State Park where equestrian activity is allowed. It's a wide trail, which makes it suitable for horses.

2. Walker Ranch Loop: This Walker Ranch Loop appeals to horse riders due to its varying terrains and beautiful canyon views. It's definitely a favorite among equestrians exploring Placerita Canyon State Park.

3. Los Pinetos Trail: Traversing through diverse landscapes, Los Pinetos Trail guarantees a memorable horseback riding experience. It meanders through forests and canyons, making it a ride to remember.

4. Placerita Creek Trail: Offering serene views of the babbling Placerita Creek, this trail is a hit with horse riders. Its predominantly flat terrain and ample shade make it convenient for a leisurely ride. 

5. Waterfall Trail: Despite being relatively short, the Waterfall Trail allows for a quick yet scenic horseback ride. Riders can enjoy the sight of a seasonal waterfall as they traverse this trail. 

For those seeking a serene spot for horseback riding in Santa Clarita Valley, Placerita Canyon State Park offers an ideal destination. Its varied landscapes and panoramic views ensure a unique, unforgettable experience for equestrians. Each trail has its own distinctive character, making every ride a new adventure.

Sources: 

Santa Clarita Valley Historical Society (scvhistory.com)

Los Angeles County Parks and Recreation Department (parks.lacounty.gov) 

Placerita Canyon State Park website (placerita.org)

Before visiting, it is always recommended to check specific park rules and guidelines as they are subject to change.

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.