Subject pronoun is a grammatical form.
Subject pronoun is a subcategory of pronoun, which is a subcategory of noun.
Subject pronouns function as the heads of pronoun phrases or noun phrases.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.