The nominal forms all end in –s except for "mine". In some Southern U.S. and New England vernacular dialects, all nominal possessive pronouns end in –n, just like mine, as in That book is hern
(but not “That's hern book”) and Those cookies are ourn
Although forms such as hisn and hern are highly socially stigmatized, from a strictly linguistic standpoint these forms reflect a natural phenomenon in the development of all languages and dialects: Irregular patterns tend to be regularized, thereby eliminating exceptions to language “rules.” Further, hisn, hern, ourn, yourn, and theirn have a long history in English. They arose in the Middle English period (c. 1100–1500) by analogy with mine and thine, forms that are older than my and thy and that can be traced to Old English (c. 449–1100). Originally, my and thy were used before nouns beginning with consonant sounds, as in my book, while mine and thine were used before nouns beginning with vowel sounds, as in mine eyes—as a and an still are. This distinction persisted into the 18th century. But as nominal pronouns, mine and thine remained unchanged. This invariant use of –n led to its use for all nominal possessive pronouns (except its, which usually is not used nominally, as in That book is its).
In fact, these –n forms may be older than the current standard –s forms, which arose late in the Middle English period, by analogy to his. Most likely, hern, ourn, yourn, and theirn originated somewhere in the central area of southern England, since they can still be found throughout many parts of that region. In the United States, the forms appear to be increasingly confined to older speakers in relatively isolated areas, indicating that these features are at last fading from use. In some Southern-based vernacular dialects, particularly African American Vernacular English, the irregular standard English pattern for nominal possessive forms has been regularized by adding –s to mine, as in That book is mines. See Note at an1.