Now, we know this for certain that Big Mom and Kaido will form an alliance in the near future. I’m now going to tell you a bit about why these two were working together in the past as well. When Hina and Vice Admiral Monkey D. Garp were in a conversation, Garp gave us the details about Rocks pirates. This crew existed more then 40 years ago in the New World and this was way before the era of Gol D Roger.
It was also revealed that Garp is known as the hero of the marines due to Kaido and Big Mom. And since this detail was revealed right along with Rocks it hints about that Kaido and Big Mom were the crewmates at the Rocks pirates. I can say that with certainty because of a valid reason. The reason being that in order to control all the seas supremely Rocks pirates needed a huge crew and also members that are extremely powerful. Thus, it hints that this pirate crew had to have crew members like Big Mom and Kaido. Thus, proving that they were also an alliance 40 years ago as the crewmates of the Rocks pirates.