Any horse trials?

I wouldn't recommend trying to bring horses up there

No horses. Literally no way to enter given turnstile entrance. Plus, no parking, as someone else mentioned.

Parking would be troublesome for horses. Much better places near by

The trails are too steep for a horse.

Pena adobe (lagoon valley) is good for horses

Pena Adobe in Vacaville is a good location for horses, not Rockville Hills Park though.

Nope. Try Skyline Park in Napa

No

I'm not sure specifically for horses.