Vishnu and Shiva are the three mighty Gods forming the Hindu Trinity.
Of these Brahma and Vishnu are mentioned in Vedas. There is no mention
of Shiva. But there is the mention of a Deity by name Rudhra to whom prayers
are addressed for saving men from the effects of lightning and for protecting
the sacrificial fire. He was considered as a mighty violent Deity having
power over fire (Agni) living in Himalayan heights among the clouds.
Although Rudra was a mighty God he
was not on a par with Indra, the Deity in Chief of the Aryans. Obviously,
Shiva did not find a place in the early Vedic literature as he was a non
Aryan God belonging to a different cult. He was recognised only at a later
period as one of the Gods in the Vedic literature along side of Rudra.
We find this in the Kaivalyopanishad. "Sa Brahma, Sa Vishnu, Sa Rudra,
Sa Shiva Sokshara" (He is Brahma, He is Vishnu, He is Rudra, he is Shiva
and he is indestructible). Evidently when this upanishad came into being
the merger of Rudra with Shiva had not taken place.
In the seals of the ancient Indus valley
there are figures of Yogis and these may be correctly surmised to be representations
of Shiva. There is however no seal where, in any, figure, he is seen riding
a bull. Even in the seal depicting the Yogi surrounded by animals, the
lion and the bull are conspicuous by their absence. Perhaps the Lord of
Yoga got his consort riding a lion and his own mount the bull only at
a later date.