Short answers, pulled from the story.
The Keśin Hymn is hymn 10.136 of the Rigveda, one of ancient India's sacred collections of Vedic Sanskrit hymns. It describes ascetic wanderers called the Keśin, who were homeless, clad in yellow tatters, and believed to travel with the wind and consort with gods and wild beasts.
Karel Werner identifies the Keśin Hymn as the earliest written evidence of yogis and their spiritual tradition. The munis depicted in the hymn experience altered states of consciousness and possess the power to soar on the wind, which scholars read as early evidence of yogic practice.
The Keśin drank from a cup shared with Rudra, the fierce Vedic storm deity. The drink is described as poisonous to ordinary mortals. Scholars disagree on whether this refers to an actual intoxicant or to a form of spiritual ecstasy produced by asceticism.
Yāska, writing around 500 BCE, interpreted Keśin as referring to the sun or the sun god Surya. Sāyana in the 14th century supported this view, as did H. H. Wilson and M. Bloomfield. Hermann Oldenberg took a different position, arguing the hymn described orgiastic Vedic practices and drunken rapture.
Karel Werner contrasts the two types: the Keśin were silent wanderers who never settled, while the Rishi were teachers who lived in huts and instructed others. Both were solitary figures, but the Keśin were defined by constant movement rather than by instruction.
The Keśin Hymn is considered a precursor to the Rudra-Siva tradition in Hinduism, primarily because of its mention of Rudra and the shared ascetic cup in the hymn's final verse. This connection links the ancient long-haired wanderers to one of Hinduism's most enduring divine lineages.