The affix ṅīp comes after the word rātri in the chandas (Vedas) and in denoting a Name except the affix jas (1st - Nominative Case Plural) is added.,
Thus या रात्री सृष्टा, रात्रीभिः (3rd pl); (Rigveda X. 127. I. I. 35. 1) but in the nominative plural we have रात्रयः, the regular plural of the word रात्रि, as in the sentence यास्ता रात्रयः, instead of रात्र्यः ॥
Vart:- According to Katyayana, nip is not added, not only when we apply the nominative plural termination जस्; but in all other terminations beginning with जस् also. Thus रात्रिं सहोषित्वा; here in the accusative singular case also the ङीप् is not employed.
But how do you explain the form रात्र्यः in the following तिमिरपटलैरवगुण्ठिताश्च रात्र्यः; This 'ratrya' is the nominative plural of 'ratri' which is formed by the feminine affix nip; and not by nish. The forms evolved by adding ङीष् or ङीप् are one and the same, except with regard to accent; that formed by nish has udatta accent on the final: the other has it on the initial. The word रात्रि is formed by the krit affix त्रिप् (Un. IV. 67), and therefore, it is a word which is governed by 4.1.45 because it is a word falling in Bahuvadi class, by virtue of the general subrule \a word ending with the vowel इ of a krit affix, other than क्तिन् belongs to Bahuvadi class; some say that every word ending in इ, if it has not the force of the affix क्तिन् belongs to this class\. Therefore it takes ङीष् in forming the feminine.,