Last chapter irevealed new information about the Rocks Pirates. Whitebeard, Kaido, Big Mom and Shiki and were once members in the crew.
Their captain was Rocks D. Xebec who was considered Gol D. Roger’s first and greatest enemy.
It took both Roger and Monkey D. Garp to finally kill Rocks.
According to Sengoku, the Rocks Pirates members were very violent and would kill friends and foes alike, and their lack of camaraderie caused crewmembers to kill each other constantly even while at sea.
Here Whitebeard mentions about someone who injured him in the past.
While it could be Roger, Shanks and Roger look nothing alike and Whitebeard was too close to Roger to mention him as ‘that guy’.
To bring together such a rowdy bunch, Rocks D. Xebec had to rule with an iron fist. He might have gathered his crew in the first place based on his might alone. Submit willingly, or get broken and then join. Something later emulated by his apprentice Kaido.
I think Rocks D. Xebec was the guy Whitebeard was talking about. That means Shanks is his son. Which is why Shanks is wary of Blackbeard, who wants to fulfill his idol Rocks D. Xebec’s dream.
*Theory by pice0fshit