A major rise of Lord Shiva in the pantheon to a major status and position in Hinduism was mainly because of his large scale facilitation and his personal identification with a host of Vedic deities like Vayu, Prajapati, Indra, Agni and many others.
It is to be noted that Agni and Rudra have a close relationship according to the Hindu religious scholars and pundits. Lord Rudra’s gradual act of improvement into the later character as Lord Rudra-Lord Shiva is mainly due to the identification between Lord Agni and Lord Rudra in the Vedic literature. An important early text on etymology named Nirukta mentioned the identification of Lord Agni with Lord Rudra is largely explicitly. The text mentioned as “Agni is called Rudra also.”
It is to be noted that some epithets of Rudra is mentioned in the Śatarudrīa like Tivaṣīmati (“Flaming bright”) and Sasipañjara (“Of golden red hue as of flame”) that largely suggests the great fusing of the two said deities. It is to be remembered that Lord Agni is stated to be a strong bull and Shiva is said to possess a Bull as his personal vehicle named as Nandi. It is to be remembered that Lord Bhairava is considered as Agni and a form of Lord Shiva.
The Vedic Indra is a continuation of the Puranic Shiva according to certain religious theory. Both Indra and Shiva are largely known for having a particular thirst for Soma. These Lords are greatly associated with the Supreme Self, the Aum sound, male fertility, rivers, fierceness, mountains, fearlessness, transgression of established mores and warfare.