Onagadoris, and they are definitely real. Their feathers get so long due to a non-molting gene, and i believe the record held for longest tail is 45 feet!
Yes, they're real. As we know them, they are called phoenix. There are people who breed them for the sole purpose of amazingly long tail lengths. That can be accomplished with the proper breeding, proper lighting, proper feed, setup, so on and so on. It takes an incredible amount of patience.