In Black Clover, all the squads' names consist of a colour followed by an animal, like Black Bulls, Coral Peacocks etc. However, Golden Dawn is an exception. Why?
We haven't gotten an actual reason in-universe. It is likely that a previous captain simply liked the name ; we know that captains can decide to rename their squad (Aqua Deer was called "Grey Deer" under the captaincy of Julius).
However, the meta reason is that the squad probably took inspiration from the Hermetic Order of the Golden Dawn, a real-life organization dedicated to the study of the occult and paranormal. Two of three founders of the occult group were named William, much like Golden Dawn's captain William Vengeance.