Who beat Kaido?

[SSJ2]

So presumably his loss with the Rocks pirates to Roger/Garp is one of them. What about the other six? Even assuming the other 3 Yonko have beaten him, who else would be capable of defeating Kaido?

 

[Warmmedown]

Shiki?

Was he in some crew? Maybe his captain defeated him, similar to WB and Ace.

Dunno if Mihawk is strong enough (probably, considering him training with Shanks), but I can imagine Zoro one day being on par with the current Yonkou, so could have been some previous swordsman.

Could have been anyone while he was coming up, I guess. Like Luffy has been defeated twice by Crocodile and once by Kuma already (twice if you count Thriller Bark, unfair on Luffy though) and could have been defeated by so many if he crossed their path too early or by some he beat if he without his crew's help (eg Moria/Oars and maybe Lucci).

If the 7 counts marines, then probably an old Admiral or Vice-admiral or even someone lower-ranked (Luffy got defeated by Smoker). We don't really know much about the top marines from that time and what devil fruit powers they had.

 

[Symbiote]

If he was caught 18 times, doesn’t that technically count as a loss?

 

[Captain Cadaver]

Possibly some of the other big names of Roger's era back when he was just a rookie in Rocks' crew. Other than Whitebeard, who it's confirmed did a lot of damage to him last time they fought (as well as possibly Shanks given they had a conflict just before Marineford), it's likely he was beaten by the Yonko crews at various points if it was during his prime, given he definitely wouldn't have lost 1 V 1 with what's revealed not long after in that chapter.

If he was caught 18 times, doesn’t that technically count as a loss?

Considering what's revealed just after this, he likely let himself be captured to test if he could actually be killed rather than losing outright.