John Hay quote: “At my door the Pale Horse stands To carry me to unknown lands.”

“He was Death, and he'd ridden in on a pale horse…”

Sherrilyn Kenyon (1965) Novelist

Source: Invincible

“You can’t blame me,” Ascanio said. “Anybody in my place would be concerned. You don’t even have a proper horse. You’re riding a mutant equine of unknown origin.”
“Don’t disrespect my donkey”

Ilona Andrews American husband-and-wife novelist duo

Source: Magic Breaks

“There are things known
and there are things unknown
and in between are the doors.”

Jim Morrison (1943–1971) lead singer of The Doors

Aldous Huxley, using the term "the doors of perception" which originated with William Blake in The Marriage of Heaven and Hell. It is sometimes credited to Morrison because he cited it in interviews as the inspiration for the name The Doors and without always crediting Huxley as the source.
Misattributed
Variant: There are things known and there are things unknown, and in between are the doors of perception.
Source: Letters from Joe

“Like you, an alien in a land unknown,
I learn to pity woes so like my own.”

John Dryden (1631–1700) English poet and playwright of the XVIIth century

Aeneis, Book I, lines 889–890.
The Works of Virgil (1697)

“In the universe, there are things that are known, and things that are unknown, and in between, there are doors.”

William Blake (1757–1827) English Romantic poet and artist

“There are things known and there are things unknown, and in between are the doors of perception.”

Aldous Huxley (1894–1963) English writer

Aldous Huxley, using the term "the doors of perception" which originated with William Blake in The Marriage of Heaven and Hell. It is sometimes credited to Morrison because he cited it in interviews as the inspiration for the name The Doors and without always crediting Huxley as the source.
Misattributed