The forms sakhī and aśiśvī are irregular forms ending in ṅīṣ found in secular (or vernacular as opposed to Vedic) Sanskrit.,
Thus सखीयं मे ब्राह्मणी, and नास्याः शिशरस्तीति = अशिश्वी ॥ Why do we say in vernacular Sanskrit ? Observe सखा सप्तपदी भव and अशिशुमिव मामयं शिशुरभिमन्यते ॥
Sometimes we find these forms in the Vedas also, though rarely, as आधेनवो धुनयन्तामशिश्वी ॥ सखी सप्तपदी भव ॥,