The Negative Particle is optionally retained unchanged in nago when it does not mean a living animate being.,
Thus नगाः or अगाः 'trees', or 'mountains', literally 'what do not move'. The affix ड is added to गम् ॥ Why do we say when not referring to animate beings. Observe अगः वृषलः शीतेन ॥ No alternative form is allowed here.,