Ingethel, the daemon that is "responsible" for the Horus Heresy, is actually a Daemon Prince from Cadia.
Ingethel, also known as Ingethel the Ascended, is a Daemon Prince of Chaos. Ingethel was originally a native to Cadia and was appointed by the Gods of Chaos to guide Lorgar during The Pilgrimage. After Ingethel revealed the Primordial Truth to Lorgar, she herself ascended to Daemonhood following 10 human sacrifices
At that time, Cadia had been newly recolonized by the Imperium.
Before Imperial colonization, Cadia was the home of a lost fragment of humanity that worshipped the four Gods of Chaos. This society was encountered by the Word Bearers Legion forty years before the Horus Heresy (...). It was on Cadia that the Word Bearers' Primarch, Lorgar, first met Ingethel the Chosen and became inducted into the service of the Foul Powers. This civilization was eventually wiped out by Cyclonic torpedoes.
N'Kari, Bubonicus and Doombreed
In the ancient lore (2nd edition), there were in the Chaos Codex a quatuor of Daemon Princes that were overpowered. Two of them was N'Kari and Bubonicus:
In Codex: Chaos (2nd Edition), N'Kari is stated as a Daemon Prince of Slaanesh instead of a Keeper of Secrets. He was born on an isolated, backwater world which had been trapped in the centre of a violent warp storm for millennia, incidentally on the same backwater world as the Daemon Prince Bubonicus.
N'Kari is now a Keeper of Secrets, hence a Greater Deamon.
Lothar (now known as Bubonicus) was a mortal Champion of Nurgle, born on a backwards planet which remains cut off from the Imperium by warp storms. Raised to daemonhood, he now rules a daemon world within the Eye of Terror, also named Bubonicus.
During the Horus Heresy, Bubonicus was one of many daemons summoned by Erebus, First Chaplain of the Word Bearers, by a dark ritual conducted during the Battle of Calth
A third of these Daemon Princes is Doombreed
Doombreed was one of the first of Khorne's servants. His True Name was long forgotten, remembered only on a scroll of dried skin, but he was once human, a mighty warlord who led armies which ravaged entire nations on Terra long ago in the Age of Terra around M1/M2.
The fourth Daemon Prince from the Codex was M'Kachen, but I don't know anything about its origin, and is now considered a Lord of Change.