Because he always nourishes all crcatures and sports also with them, and because he is their lord or master, therefore is he called Pashupati (the lord of all creatures).
सर्वथा यत्पशून् पाति तैश्च यद् रमते सह। तेषामधिपतिर्यच्च तस्मात् पशुपतिः स्मृतः॥