For the Tibetans, siddhas are above all heroic figures of the Buddhist path to Enlightenment. Many of the spiritual lineages of Tibetan tantric (Vajrayāna) practice trace their origins to named Indian mahāsiddhas. The Tibetans borrowed the concepts of siddha and mahāsiddhas from the Vajrayāna Buddhism of North India, and translated the terms into Tibetan. Within a couple of centuries, Tibetans were describing some of their own tantric practitioners as siddhas (Tib. drubtop) and mahāsiddhas (Tib. drupchen).
Holy Madness: Portraits of Tantric Siddhas p. 36-47