Matsyendranath: guru to Gorakhnath (below)
Here is a powerful quote attributed to him and that explains the core of yoga philosophy even today.
The four varna (castes) are perceived to be located in the nature of the individual, i.e. Brahmana in sadacara (righteous conduct), Ksatriya in saurya (valor and courage), Vaisya in vyavasaya (business), and Sudra in seva (service). A yogin experiences all men and women of all races and castes within himself. Therefore he has no hatred for anybody. He has love for every being.
— Gorakhnath, Siddha Siddhanta Paddhati III.6-8
Many legends are attributed to him, as evidence of his yogic prowess, usually feats to show how he aided is disciples or devotees in difficult circumstances. Many of these stories also reflect the fact that he was regarded as a great healer.
His origins were very humble. Though there are different stories of his origin, all have a common detail: he was discovered as a baby left on a dung hill. And he was the direct disciple of the powerful yogi Matsyendranath, after whom the famous spinal twist pose in yoga is named.