A livestock lien is different from a mechanic's lien. There is no form that I know of for a livestock lien. The lien against the horse is automatic. The police are correct that the matter is civil. To enforce your lien you will have to file a lawsuit. That can be costly and difficult since there are some difficult technical steps that must be followed.
I don't know if Woodside is close to Redwood City, but there is a law firm there that has good information on livestock liens. Take a look at the link below, and maybe look for an attorney in your area that has experience with these matters.