Primordial Monsters (Adventure Time) such as Orgalorg and Hunson Abadeer, are such ancient beings that they predate literally everything, and even nothing.
Hunson Abadeer (Adventure Time) has existed since before time and before everything, when "there were only monsters" like Orgalorg.
The Lich King (Adventure Time) is a primordial monster that not only predates existence but even the time before that.
Illyria (Angel) is one of the Old Ones, an ancient race of pureblood demons who lived during the Primordium Age.
Juno, Minerva, and Jupiter (Assassin's Creed) are members of the First Civilization, beings who not only predate the human race, but created it.
The First Evil (Buffy the Vampire Slayer) came into existence long before the universe was created, and will remain long after its end.
Ulthane (Darksiders) is one of the Makers, a race that predates Heaven and Hell.
Contrary to initial belief that he was a creation of Bibidi, Majin Buu (Dragon Ball) has existed since the dawn of time itself.
Jack of Blades (Fables) has existed for thousands of years, and was already ancient by the time humanity came into existence.
Dendar, The Night Serpent (Forgotten Realms) is the original primordial nightmare and as such is as old as time and conscious thought itself.
The Nibblonian race (Futurama) was already 17 years old by the time of the Big Bang.
The Precursors (Halo) were a race of beings believed to be even older than the universe itself.
Ophis (Highschool DxD) formed from the nothingness of the dimensional gap before time began.
The Elder God (Legacy of Kain) predates recorded history and claims to be the "engine of Life" itself.
Known as the First and the Fatherless, Tom Bombadil (The Lord of the Rings) is the oldest being in all of middle earth and beyond, having already existed long before even the creation and awakening of the Ainur by Eru Illuvatar.
The Dweller-In-Darkness (Marvel Comics) is so ancient he predates not only the big-bang but even the universe before that one.
Galactus (Marvel Comics) predates the Big Bang.
Ajimu Najimi (Medaka Box) is trillions of years older than the universe and everything else.
The Shinju (Naruto) is the primordial source of chakra in the Ninja World.
Like all Obyriths, Pale Night (Planescape) has been around long before the multiverse ever existed, as she and her kind have been destroying countless realities outside of the proper multiverse before it was even born.
Chip/Light Gaia (Sonic the Hedgehog) is a primordial deity who embodies day, light, and rebirth...
...while his counterpart, Dark Gaia, is a primordial energy being that symbolizes darkness, night, and destruction.
The Archangels (Supernatural) predate the universe itself, battling The Darkness alongside God.
Eve (Supernatural) predates the angels themselves, and is obscurely related to the Leviathans.
The Leviathans (Supernatural) are the very first beasts ever created by God, and are as old as time.
Cybele (Valkyrie Crusade) is a primordial goddess that predates concepts such as life and death, being the one that created life itself in the first place.
Eden Maker (Valkyrie Crusade) is a primordial goddess and one of the creators of the Celestial Realm.
Birthday (Valkyrie Crusade) as the embodiment of the celestial realm's birthday, is as old as the celestial realm itself.
Gilgamesh's (Valkyrie Crusade) age is virtually immesurable, as she is eons years old.
Lord of Nightmares/L-sama (Slayers)
Hell (Nancy in Hell) is a semi-sentient realm that was originally the Primordial Darkness before God brought light, having taking shape feeding off of the suffering of the banished Lucifer.