The Rocks Pirates were a legendary and powerful pirate crew that sailed the seas 38 years before the current storyline. They were led by Rocks D. Xebec and considered to be the most dangerous and strongest pirate crew of their time.
The Rocks Pirates ruled the seas before the rise of Gol D. Roger. They were without a doubt the strongest pirate crew of their time. Aiming to become the strongest military force in the world, they had at least three of the four future Yonko.
They were the most threatening organisation to the World Government.
Eventually, it took the combined strength of Gol D. Roger and Monkey D. Garp to defeat them.
I gave a look at Garp’s wikies and found out, he was chasing Shakky (Rayleigh’s wife) about 40 years ago. During the Sabaody Arc we get to know about it and Shakky said she just quit being a pirate 40 years ago to open her bar. The Marine may assume she’s actually dead and that Garp may killed her.
The odds are real that she was someone in the Rocks Pirates. In support of this theory, recently Oda himself hinted that Shakky might be part of the Rocks Pirates. Here’s what he said, answering a fan’s question:
Reader: “Thinking about “pirate”, “40 years ago” and “Garp”, the one who comes to mind is Shakky! Do Shakky and Rocks have something to do with each other?”
Oda: “Huh…!! You remember things over 40 volumes ago!? N… no, they don’t!! Lalalala [TL/N: It’s the sound in japanese someone makes who got caught]”