There is no way harald and Kaido aren’t related in some way
Loki is 63, Kaido is 59.
1154 covers between:
-63, 56, and 48 years ago, a 15 year gap
-Kaido was born 59 years ago, when Loki was 4
-So, Harald met Loki before having Kaido.
-Estrid died before Harald met Kaido's mom, and Ida was still alive
-Kaido was born between the Food/Famine flashback 63 years ago, with the Human ships feeding Elbaph, and then 56 years ago, the Jarul/Harald conversation about Giant Shipwrights, Galleyra.
-Harald was trying to spread diplomacy between different Kingdoms/Races
-Harald is an Oni, but despises his Oni genetics, yet still has horns here
Chances are Harald met Kaido's Mom while researching the Oni Race/trying to branch out to the Onis. Since he has Oni blood, Harald was researching his heritage.
Since Harald still has his horns here, I think his horn removal is a result of his encounter with Kaido's Oni-Mom.
Elbaph's been very racism focused so far, between Rodo treating Nami like a Barbie Doll, to Loki hating humans at first, to Hajrudin being treated like crap over his mom.
IMO, Kaido's Oni Mother was hated by the Onis for birthing a mixed-race child, and killed for it. Kaido was sent adrift to sea, either by his mother to escape, or by the Oni who killed her to kill him via burial by sea.
Harald returned to find that the woman he had loved and her son were killed by the other Oni, and, Harald murdered all of them, before ripping his own horns out in disgust.
Harald ripped his horns out over Kaido's Mom dying, sadly, I believe