Why we call Shiva as Pashupathi?
The name has also been applied to a figure, probably a god, depicted as sitting among animals, discovered in the context of the Indus Valley Civilization. This god has been suggested as an earlier form of Rudra. Parallels have also been drawn with the Celtic Cernunnos.
In Shiv Puran, this has been explained in details.
This is the event at the time of Tripur Sanhar.
That time Lord Shiv Shankar has been made and declared by Pashupati by Vishnu Bhagawan & all other deities (DEVATA).