rival - noun -plural noun: rivals
1. A person or thing competing with another for the same objective or for superiority in the same field of activity.
"he has no serious rival for the job"
synonyms: competitor, opponent, contestant, contender, challenger; More
rival - verb
1. Be or seem to be equal or comparable to. Hence my statement
I'd add Moriah as he was said to be his rival once upon a time. [
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] Those many characters who went 'toe-to-toe" against other characters and were "tough enough" have/had grounds to claim rivalry with said character.
For a moment stop focusing on the tree and you'll see the forest.
Both translations are implying Kaido and Moriah were once peers only varying in legitimacy ( one rumor vs other reality). Originally, the idea was to
make a educated guess on who is bears likely hood on having bested Kaido. The
situation(s) behind the loss is as irrelevant as his or his opponents supposed level. If we are to make Luffy's L count, Crocodile appears on the list (twice in fact) as for the duration of Alabasta he was indeed Luffy's rival.
Manga-wise Moriah is the only other enemy who Oda saw fit to contrast his battle prowess to.