Cal. Rev. & Tax. Code § 5782

On forms provided through the office of the assessor, the owner of a racehorse either in person, through his representative or by mail, shall report the tax due. The reports required by this section may be filed with the tax collector of the county in which the racehorse had its domicile. The reports shall be filed on or before 5:00 p.m. on the day the tax due becomes delinquent.