"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!"
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.
The Cosmic Entity (Supernatural) is a nameless 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 as 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.