The affix ini (in) comes in the sense of matup after a prātipadika (Nominal Stem) ending in man or ma when the whole word so formed means a Name.,
Thus प्रथिमन् + इनि = प्रथिम् + इन् 6.4.144 = प्रथिमिन् fem. प्रथिमिनी ॥ So also दामिनी ॥ Similarly होम, होमिनी, सोमिनी ॥ Why do we say, 'when meaning a name'? Observe, सोमवान्, होमवान् ॥,