Magical creatures that have mostly left Earth to live in the Umbra because of Sleeper unbelief making the physical world hostile to their existence. Most resemble legends & popular notions of magical creatures. Many Bygones have become Umbral spirits but some retained their physical form by living in Horizon Realms or places on Earth where there is a reliable supply of Quintessence & little observation by Sleepers. Paradox takes a heavy toll on Bygones when they are on Earth although some still return on occasion. Published Mage books have mentioned Marauders bringing Bygones to Earth to disrupt the Consensus.
The Bygone Bestiary is a good source of Bygones and options to create your own. Also see powers for aquatic Bygones in Blood-Dimmed Tides pg. 101. 5 are found on pg. 91 of Book of Madness.