3 things are considered when it comes to immortality. Death happens either by aging, by disease or by physical trauma. Having a situation that only negates some of these isn't complete immortality. Simply put, an immortal doesn't die. The Ope Ope which is said to grant immortality means the person affected would not die.
No he can regenerate himself as long as his body doesn't sustain too much damage. If all of his bones are crushed he will die, even in that form.
no, the buddha is a mythical zoan, its not a part of any "family" also, since choppers fruit encompasses, giant forms aswell, i doubt there are any other human fuits.
Flamingo was looking to obtain and use the immorality fruit himself, and Kaido was closely affiliated with him
It may be impossible to kill him, but it should be possible to immobilize him. I.e. frozen for eternity, chopped in pieces by the Ope Ope no Mi, etc.
Oni are said to be extremely strong, even invincible. If he has a mythical fruit maybe it could be that.
I think when they're talking about creature, they're simply talking about his anatomical or biological make up in comparison to all other creatures.
But there's a huge chance that WB was the one who inflicted that scar on Kaido, shows you he isn't invincible
Agreed there, he's definitely not invincible. He might be from the Oars race and more so since Moriah stole his body in order to defeat Kaido I think with the exception of the fact that he lacks the teeth that Oars have been shown to have so far.
I was about to say that maybe Kaidou was an animal that ate human-human fruit also
I was thinking that maybe Kaidou is a real dragon that ate human-human fruit model giant or something. But human-human model oars works too 
The moster that ate Shanks arm wasn't a seaking.
I was thinking about the Monsters Oneshot too. But we have to remember that even though Oda confirmed that the Ryuuma in Thriller Bark is the same Ryuuma from Monsters it doesn't make Monsters part of One Piece - at least not yet. Oda might use it in Wano arc or he might not. And if he uses it he might change some things - like the place where Ryuuma slayed the dragon. Only thing we know for sure is that there is a story told that Ryuuma slayed a dragon, and I believe for that we can take Monsters at confirmation. And I think it would be very fitting if Ryuuma killing the dragon would have something to do with Wano. Though if it had anything to do with Kin's grudge against dragons is hard to say - afterall Ryuuma killed the dragon generations ago.Ryuma did NOT slay the dragon at Wa, and neither did the dragon terrorize Wa. He did so at another island, in which he happened to be.
Furthermore, when Flare, the girl who had her village burned to a crisp by the dragon Ryuma later slayed, was absorbed in her fear of dragons, Ryuma showed no ‘hate’, ‘grudge’, ‘anger’, ‘fear’ or any such feeling. Instead, he was wondering about its scariness.
Someone who had suffered the tragic attacks of a dragon or dragons at Wa would not be asking such question since the answer to it would have been known through his experience. So the surmise about the possibility that the people of Wa ‘hate’ dragons because their country gets attacked by dragons does not hold.
If Kaido has horns because he is of a different race, I would be more willing to believe he would share his race with Moriah, not Oars. Perhaps Oars could be a hybrid of a giant and the horned human race.
I always felt that Moria is from the same race as Magellan, and maybe even Hannyabal. They seem to come straight out of Halloween, with a horror theme going on.
He could be a hybrid/abnormal genetic mutation like Blackbeard.