Rudra-prasava-varṇana
The Manifestation and Naming of Rudra / Nīlalohita
विवेश तत्तदा ह्यग्निं तस्मात्पशुपतिस्तु सः / यस्मादग्निः पशुश्चासीद्यस्मात्पाति पशूंश्च सः
viveśa tattadā hyagniṃ tasmātpaśupatistu saḥ / yasmādagniḥ paśuścāsīdyasmātpāti paśūṃśca saḥ
Then that radiance entered into Agni; therefore he became renowned as Paśupati. For he became both Agni and paśu, and because he protects all paśu.