California's Vehicle Code (VEH) would still allow for a DUI charge when riding a horse. According to urban legend (and a lot of dubious websites), there are cases where Californians have been convicted of a DUI on a horse, though I haven't been able to find reputable sources for those claims.

VEH 1.670 [0]

> A “vehicle” is a device by which any person or property may be propelled, moved, or drawn upon a highway, excepting a device moved exclusively by human power or used exclusively upon stationary rails or tracks.

VEH 11.12.23152(a) [1]

> It is unlawful for a person who is under the influence of any alcoholic beverage to drive a vehicle.

[0] https://leginfo.legislature.ca.gov/faces/codes_displaySectio....

[1] https://leginfo.legislature.ca.gov/faces/codes_displaySectio...