Genghis Khan (born Temüjin), is reputed to have been born with a blood clot in his right hand.
Then Yisügei-ba’atur plundered Temüjin-üge, Qoribuga, and other Tatars. On his return, Lady Hö’elün was pregnant. They were at Deli’ün-boldaq on the Onon when Chinggis Qahan was born. At the time of his birth, he held in his right hand a clot of blood as big as a knucklebone. Born at the time of the capture of the Tatar Temüjin-üge, they thought to name him Temüjin.
The Secret History of the Mongols, trans. Urgunge Onon, page 57
I've seen vague statements of this being an auspicious sign for his birth, others claiming it to be a sign of leadership, and others taking it for a sign of bloodlust. The latter explanation I seem to see least commonly, and looks a bit like a westernized misunderstanding at first blush, but it does seem to be the interpretation taken later in The Secret History of the Mongols:
Accidentally issuing from my hot [womb],
this one was born holding a black clot of blood in his hand.
Like the qasar dog gnawing on its own afterbirth,
like a panther attacking on a rocky mountain,
like a lion unable to control its anger,
The Secret History of the Mongols, trans. Urgunge Onon, page 69
So, are there other sources to attest to the meaning of being born holding a blood clot? What was the traditional meaning of it in the Mongolian traditions of the time?