"Eldest, that's what I am...Tom remembers the first raindrop and the first acorn...he knew the dark under the stars when it was fearless — before the Dark Lord came from Outside..."
― Tom Bombadil (LOTR)
"Stop. I have learned much from you. Thank you, my teachers, and now for your education. Before there was time, before there was anything, there was nothing, and before there was nothing, there were monsters. Here's your gold star!"
― Sweet P/The Lich (Adventure Time)
"I lived ten thousand lifetimes before the first of your kind crawled out of the mud."
― Vaatu (Avatar: The Legend of Korra)
"This is one little planet, one tiny solar system, in a galaxy that's barely out of its diapers. I'm old, Dean, very old, so I invite you to contemplate how insignificant I find you."
The Soul King (Bleach) is a being that emerged from the chaotic old world, and whose power was used to reshape it into the current one.
Baraggan Louisenbairn (Bleach) was one of the original Hollows, born in the old world.
Ikomikidomoe (Bleach) is an ancient Hollow, one of the first Menos, born in the old world.
Ichibē Hyōsube (Bleach) is a Shinigami from the old world, and is one of the few beings who knows about its very existence.
Contrary to the initial belief that he was a creation of Bibidi, Majin Buu (Dragon Ball series) has existed since the dawn of time itself.
Ophis (Highschool DxD) formed from the nothingness of the dimensional gap before time began.
Ajimu Najimi (Medaka Box) is trillions of years older than the universe and everything else.
The Shinju/Ten-Tail/Jubei (Naruto) is the primordial source of chakra in the Ninja World.
Lord of Nightmares/L-sama (Slayers)
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.
The Nibblonian race (Futurama) was already 17 years old by the time of the Big Bang.
The Dweller-in-Darkness (Marvel Comics) is so ancient he predates not only the Big Bang, but even the universe before that one.
Eternity (Marvel Comics)
Galactus (Marvel Comics) predates the Big Bang.
God of the Symbiotes, Knull (Marvel Comics) is an ancient malevolent deity who seeks to expand the darkness all across the universe by conquering, destroying and corrupting every life form.
Tikki (Miraculous: Tales of Ladybug & Cat Noir) is the abstract embodiment of creation, being the first being to come into existence and having lived since the dawn of the universe itself.
Hell (Nancy in Hell) is a semi-sentient realm that was originally the Primordial Darkness before God brought light, having taken shape by feeding off of the suffering of the banished Lucifer.
Juno, Minerva, and Jupiter (Assassin's Creed) are members of the First Civilization, beings who not only predate the human race, but created it.
Ulthane (Darksiders) is one of the Makers, a race that predates both Heaven and Hell.
Davoth, the Dark Lord of Hell (Doom), was the first of the Father's Primevals and is believed to be over a decillion years old.
Jack of Blades (Fables) has existed for thousands of years, and was already ancient by the time humanity came into existence.
The Ancients (God of War) are a race of elemental golems said to be the lingering fragments of the primordial being and progenitor of the Jötnar, Ymir, making them as old as the beginning of time itself.
The Precursors (Halo) were a race of beings believed to be even older than the universe itself.
The Elder God (Legacy of Kain) predates recorded history and claims to be the "engine of Life" itself.
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.
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 immeasurable, as she is eons years old.
Illyria (Angel) is one of the Old Ones, an ancient race of pureblood demons who lived during the Primordium Age.
The First Evil (Buffy the Vampire Slayer) came into existence long before the universe was created, and will remain long after its end.
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, predating the angels, humans and the souls itself.
The Shadow (Supernatural) is a primordial being whose existence predates all things, including Amara, God, Death, Creation and Destruction, as he is the original emptiness/nonexistence they all eventually emerged from.
Dendar, The Night Serpent (Forgotten Realms) is the original primordial nightmare and as such is as old as time and conscious thought 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.
Nyx/Nox (Greco-Roman Mythology) the goddess of the night, one of the primordial gods (protogenoi) who emerged at the dawn of creation.
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.
The Fae (SCP Foundation) are the ancient, original race that ruled the world and reality long before the Children of the Night/SCP-1000 and the Children of the Sun ever even existed. Even predating the concept and existence of Adam and Eve.
Eru Ilúvatar (The Lord of The Rings) is the The All Powerful creator of the Tolkien universe.