So, let's discuss this matter.
Kaido and Big Mom were part of some certain organization called "Rocks/Locks". They were comrades 40yrs ago (reason why BM told Kaido "Let's get along just like the old days"). This organization lost their captain but it's 100% sure that they are making their comeback.
Before Roger came, it was their era. You may ask me why I said that BM and Kaido were part of them, well it's because the marines are talking about the 2 emperors possible collaboration and Garp inserted the "Rocks/Locks". I never saw Garp worried that much (tho still smiling while worrying), saying that if they were to make their come back, it'd be an incident too far beyond their imaginations. So, point to be taken, this group is evil. They are LEGEND and is LURKING (except BM and Kaido since they're not in hiding). Maybe perhaps, One of them was the one Oda is talking about, regarding the greatest enemy the Strawhats will face who will make an appearance in Wano.
Another one, Hina said that, that was the (real) reason why Garp was called as the hero of the marines.. so it's not about how many times he cornered Roger. If that's the real case then that must be the real deal, but it is still vague if Garp was the one who beat them since what Hina said was they lost their captain only, she did not imply anything about Garp defeating them. But, if what I think is right, then Garp is someone who can stand his ground against this group/pirate crew.
Here is what I think, any of you remember the old woman with Rayleigh in Sabaody? Shakky
https://imgur.com/a/ZwAEq9v She claims to have survived being chased by Garp. And with that feat, I mean, surviving someone as strong as Garp (in his prime), can she be one of the member of that pirate crew as well? Since it was also stated that she used to be a pirate for over 40yrs already. And Garp is someone whom they run for their lives. Made me imagine how strong really Garp is (when he's on his prime).
What do you think?