Galactus is known throughout the universe as a gargantuan eater of worlds, but his original identity was a humble humanoid named Galan:

...native of a planet called Taa, in a universe that existed before Marvel's 616 universe ever came to be. Taa was destroyed as its universe collapsed into a Cosmic Egg, but Galan survived via shenanigans involving cosmic rays, and he was reborn into the new (616) universe that was created when the Cosmic Egg exploded in a Big Bang.
The Watcher Ecce was there to observe the birth of the being who would come to be known as Galactus, who briefly existed as Galan in his previous body, before the cosmic power consumed him entirely:



Much later, Galactus showed Thor the events of his birth, including the end of his life in the previous universe. These events are all covered in Thor #168-169, and the origin story itself was reprinted as Super-Villain Classics #1 - Galactus: The Origin (including some changes to the script, such as stating the name Galan, and changing "radiation" to "cosmic rays").
In Fantastic Four #262, Odin told Galan's story at Reed Richards' trial,to an audience including the Fantastic Four, the Watcher Uatu, Shi'ar Empress Lilandra, the Gladiator, John Byrne, and a jury composed of representatives of alien races who were affected by Galactus.
Galactus is surely known as the destroyer of worlds to many trillions of individuals throughout the universe. However, he is known as Galan only to Thor, Odin, at least two Watchers, the Fantastic Four, and the others present at Reed's trial, who could perhaps number in the hundreds or thousands, depending how many victims of Galactus were represented. While that is quite a number of people who now know about Galan and his dead universe, it pales in comparison to Galactus's fame, as there is surely no one else in the universe who is known as widely as Galactus.