The word praṣṭha is irregularly formed when denoting 'a chief' or 'one who goes in front'.,
Thus प्रष्ठोऽश्व meaning a 'best horse' Why do we say when meaning 'going in front'? Observe प्रस्थे हिमवतः पुण्ये \On the sacred peak of the Himalaya\. प्रस्थो व्रीहीणाम् 'a measure of barley'.,