The affix ṇa comes after the word pantha which becomes the substitute of pathin, the sense of the affix being 'who always goes'.,
The word नित्य qualifies the sense of the affix. Thus पन्थानं नित्यं गच्छति = पान्थः as पान्थो भिक्षां याचते ॥ Why नित्यं ? See पथिकः ॥,