Yellow Jackets

These marines as likely as canon as Malal now, as the only time they’ve really been recorded was before even the Second Founding, during the Horus Heresy, which should be impossible seeing as only the 18 Legions were around them. They were referenced once in the 90s when the lore was still forming, and never mentioned since. They have a cool Yellowjacket symbol though.

Sources: White Dwarf 177, "The Yellow Jackets", pg. 30 (Chapter created by 1994 Golden Daemon Chapter Design Competition Winner Jeff Durochia)