1. The Historical Meaning of Fingers On an Oriental Dragon
In ancient Asia, the oriental dragon has been the symbol of royalty and power. The number of fingers illustrated played a key part in showing the ranking of the wearing person.
For instance, a dragon with three digits (i.e. 2 fingers and a thumb) would be available for use by the general public and the lower ranking officials.
A dragon with four digits (i.e. 3 fingers and a thumb) would be reserved for higher royalty (like princes) and certain high-ranking officials (maybe cabinet advisors).
A dragon with five digits like a human (i.e. 4 fingers and a thumb) was to be used only by and for the emperor himself.
This is something I’ve always known since I grew up in an ethnically Chinese family. It’s actually rather common knowledge because all our illustrations on traditional Chinese items (e.g. on vases, red packets, Taoist prayer banners, temple sculptures, etc) only have dragons with 3-4 digits max.
It’s also something that Ancient China soap operas would play on (e.g. where the wannabe-traitor is caught bearing an illustration of a dragon with five digits, betraying his intention to overthrow the throne).
How does that fit into the OP storyline?
2. Kaido’s Present-Day Dragon Form
Now we have seen Kaido’s present-day dragon form:
Count the digits on each hand: 4 total. Also note the form in which he held his opposable thumb.
I’ve gone through his appearances in the manga, and it is consistent. His present-day appearances as a dragon all involve only 4 digits per hand.
What could this mean? Perhaps it’s just an artistic choice that doesn’t mean anything?
I thus put this thought at the back of my head long ago.
At this stage, I also ask you to notice his hand structure and particularly how his opposable thumb is in the exact opposite direction of his fingers.
That’s where things got interesting.
3. Past Kaido Dragon Form
I then happened to just be re-reading Oden’s flashback and just noticed today that the past Kaido has a different number of digits per hand.
For the record, I reviewed and noted that Kaido’s dragon form’s hands appear to be deliberately concealed / drawn shoddily throughout the entire flashback.
However, we have this one picture which then shows his straight fingers clearly:
4 straight fingers and Oden (and sound effect Katakana) blocking where his opposable thumb should be.
Kaido is shown with 4 straight fingers in his right hand (which is near his right chest where his Oden scar is). Given the angle his thumb is supposed to be (i.e. downwards), none of those 4 fingers could be his thumb.
Quite clearly and assuming it’s not a drawing error, Kaido 20 years ago had 5 digits per hand.
This is where the symbolism starts to come together. This could be one of those symbolic easter eggs hidden by Oda that will not be addressed directly in canon (like, “Hey Queen, I noticed this morning I lost a finger. What the hell man? I left a diamond ring on it yesterday. Dammit.”). It could simply be placed in for background symbolic purposes.
It fits into the story as back then, Kaido was the undisputed power over Wano. He was also safely on the route of emperor (on his way to rule as a Yonkou for over 20 years). He was pulling the weight of an emperor. He thus had five digits, symbolising his position as emperor.
Kaido was the one power, the one emperor, above all.
4. The Missing Finger
As of Chapter 987, Kaido still has 4 digits per hand:
This ties in with his change in circumstance, as the story now is that he is not the sole dragon anymore. There now exists a second dragon (with bloodline priority rights too) that has far greater symbolic value than Kaido as the one ruler of Wano.
Kaido’s position as the sole power is also none too safe. Many a challenger have been coming for his head. Now, there are three aspiring bigwigs who may well take over him.
There is also a group of thousands of warriors who have finally assembled and can’t wait to kill him. And who have injured him, to his surprise, something which he previously thought impossible.
In any case, do note that present-day Momonosuke also only has 4 digits per hand (i.e. 3 fingers and a thumb). See this:
5. [Theory] Momonosuke’s 5th Digit
Here’s where the speculation comes in.
Assuming it’s not a drawing error, my guess is Momonosuke will suddenly be illustrated with 5 digits in his dragon form after the Onigashima war is won and Kaido toppled (or maybe killed).
This would be the symbolic illustration of Momonosuke finally taking over as the one true ruler of Wano.