Native American Livestock Assistance Act of 2018 This bill amends the Agricultural Act of 2014 to authorize disaster assistance payments to members of Indian tribes under the Livestock Indemnity Program (LIP) and the Livestock Forage Program (LFP), including for a horse that is owned by a member of an Indian tribe and is not used for commercial agricultural purposes. (The LFP makes payments to eligible livestock producers who have suffered certain grazing losses due to a drought or on rangeland managed by a federal agency due to a fire. The LIP provides payments to eligible livestock producers on farms that incurred livestock deaths in excess of the normal mortality due to adverse weather or attacks by animals reintroduced into the wild by the federal government or protected by federal law.) The bill also amends the Federal Crop Insurance Act to offer a one-time increase in the premium subsidy for members of an Indian Tribe who purchase policies for the Risk Management Agency Pasture, Rangeland, and Forage (PRF) crop insurance program for the first time. (The PRF crop insurance program is intended to help protect a producer's operation from the risks of forage loss due to the lack of precipitation.)