'Neighbor' (or 'neighbour' outside the US) is a noun and a verb. The present participle form 'neighboring' can be used as an adjective, for example, 'The country has good relations with neighboring countries.' The base form 'neighbor' cannot be used as an adjective. If you come across a combination such as 'neighbor state', for example, this is a compound noun made up of two nouns. 'Neighbor' is not an adjective here.

When using neighbor as an adjective, you'd need to use "neighbors" for a singular noun, and "neighbor" for plural: "The state of Texas neighbors four other states, as well as the country of Mexico" "The northern states neighbor Canada." To use 'neighboring' you'd have to add "is" or "are" depending on singular or plural: "Those states that are neighboring Canada are all in the North." "Texas has four neighboring states, as well as the country of Mexico."