Either one of these pens is mine.

In the above sentence, the word "Either" is 

The correct answer is ‘Distributive pronoun’. The words that are used in place of a noun are pronouns. The name of a person, place, animal, or item can be the noun that is replaced. According to English grammar, a pronoun cannot be used until and unless a noun has been mentioned or used beforehand. The pronoun that considers a member of a group separately rather than collectively is called a distributive pronoun. In the given sentence, ‘Either one of these pens is mine.’, the pronoun ‘either’ shows the functionality of a distributive pronoun.