A Various dates are given for the birth of Temüjin (or Temuchin), as Genghis Khan was named—after a leader who was defeated by his father, Yesügei, when Temüjin was born. The chronology of Temüjin’s early life is uncertain. He may have been born in 1155, in 1162 (the date favoured today in Mongolia), or in 1167. According to legend, his birth was auspicious, because he came into the world holding a clot of blood in his hand. He is also said to have been of divine origin, his first ancestor having been a gray wolf, “born with a destiny from heaven on high.” Yet his early years were anything but promising. When he was nine, Yesügei, a member of the royal Borjigin clan of the Mongols, was poisoned by a band of Tatars, another nomadic people, in continuance of an old feud.
With Yesügei dead, the remainder of the clan, led by the rival Taychiut family, abandoned his widow, Höelün, and her children, considering them too weak to exercise leadership and seizing the opportunity to usurp power. For a time the small family led a life of extreme poverty, eating roots and fish instead of the normal nomad diet of mutton and mare’s milk.
Once he was captured by the Taychiut, who, rather than killing him, kept him around their camps, wearing a wooden collar. One night, when they were feasting, Temüjin, noticing that he was being ineptly guarded, knocked down the sentry with a blow from his wooden collar and fled. The Taychiut searched all night for him, and he was seen by one of their people, who, impressed by the fire in his eyes, did not denounce him but helped him escape at the risk of his own life. On another occasion horse thieves came and stole eight of the nine horses that the small family owned. Temüjin pursued them. On the way he stopped to ask a young stranger, called Bo’orchu, if he had seen the horses. Bo’orchu immediately left the milking he was engaged in, gave Temüjin a fresh horse, and set out with him to help recover the lost beasts. He refused any reward but, recognizing Temüjin’s authority, attached himself irrevocably to him as a nökör, or free companion, abandoning his own family.
Among other things, he was able to claim the wife to whom Yesügei had betrothed him just before his death. But the Merkit people, a tribe living in northern Mongolia, bore Temüjin a grudge, because Yesügei had stolen his own wife, Höelün, from one of their men, and in their turn they ravished Temüjin’s wife Börte. Temüjin felt able to appeal to Toghril, khan of the Kereit tribe, with whom Yesügei had had the relationship of anda, or sworn brother, and at that time the most powerful Mongol prince, for help in recovering Börte. He had had the foresight to rekindle this friendship by presenting Toghril with a sable skin, which he himself had received as a bridal gift. He seems to have had nothing else to offer; yet, in exchange, Toghril promised to reunite Temüjin’s scattered people, and he is said to have redeemed his promise by furnishing 20,000 men and persuading Jamuka, a boyhood friend of Temüjin’s, to supply an army as well. The contrast between Temüjin’s destitution and the huge army furnished by his allies is hard to explain, and no authority other than the narrative of the Secret History is available.
Source; https://www.britannica.com/biography/Genghis-Khan