It is the word used by the ODA.
Kaido is called the strongest pirate in the world, and by Zoro as "The strongest in the world".
The term creature here, for superficial readers like you might sound like it only applies to animals, here is a detailed explanation:
His title was translated by some manga website as" Strongest Beast Alive ". It was a translation error because in the official VIZ translation, which is directly associated with Oda (and we have to pay to see the translation) , is declared as "On land, sea and air. He is the pirate known as the strongest Creature of all living things ".
And Oda used the word 'Saikyō Seibutsu (最強 生物): which translated as The Strongest Biological Being / The Strongest Creature Among All Beings. Seibutsu includes all living things in the world, including humans. The reason why free manga sites misspell Kaido's title is because the English word "Creature" has plural meanings, but in the original scan there is no ambiguity because the word 'Seibutsu' is the one being used, which is why Kaido was not given the title "Strongest Man" or anything like that, because he is simply not human.
Kaido's title isn't ambiguous at all in the japanese version, his title 100% include humans.
You can do a quick research on what Seibutsu mean:
https://en.wiktionary.org/wiki/せいぶつ#Japanese
living thing, life, organism, creature .
https://www.kanjijapanese.com/en/dictionary-japanese-english/seibutsu
seibutsu creature 2. seibutsu living things.
The ambiguity only come from people reading the English chapter and assuming the word "creature" is to be understood in the animal sense.
In japanese it clearly mean Kaido is the strongest living organism/thing and quite obviously humans are living things.
Kaido's title 100% put him above any human alive in the series, the only way Kaido isn't the strongest character is if his title is fake in the first place or if someone surpass him in the future.
Now that you have learned what a creature is, I think there is no doubt, even if KAIDO is of a different race than HUMANA, among all races, for example, the strongest of the human race, is WB (example), now the strongest of another species (race) TRITON is JIMBEI and there follows the logic, more among all these races and creatures) there is someone stronger who is technically called kaido, who places all of these automatically below them.
With confirmation that it is likely that Kaido can fight in water, on land, at sea and in air, he is the strongest fighter.