King's Misery when there are several bits from the novel the writer is making to please his capturer seems like a classic at this point. More classic is the trope of Necronomicon-alikes in lovecraftian stories. But I prefer The Book of Mindfulness that a character from the Black Book sitcom accidentially swallowed and became a saint, even god-like figure.

The movie Twins starring Danny Divito and Arnold Schwarzenegger plays at the drive in the anime Blood Blockade Battlefront

Good Morning Glipglorp! From the Androids and Aliens podcast by the Glass Cannon Network. It was a random bit of world-building that the players latched on to and ran with and it turned into a whole episode.

Mrs. Albert Hannaday from The Office

I didn't see this one in the list, but UNN newscast from Starcraft 2 is one of my favorites! Particularly Donny Vermillion.

Surprised that no one has mentioned Vegemorphs from PBS's Arthur yet.

The Dick Van Dike show is a great one.

The Marvelous Misses Maisel

The game from the startreck like episode on BlackMirror and Sword Art Online! Both are arguably great.

The twilight zone episode about the podcast and the airplane is amazing too.

I really like Fire and Blood.

There's so much in Infinite Jest, but I think my favourite would be "Blood Sister: One Tough Nun".

Galaxy Quest the Show inside Galaxy Quest the Movie

Do the Justice Friends from Dexter's Lab count?

As The Stomach Turns. It was a classic parody of soap operas on The Carol Burnett show, and as someone from that era i can vouch that it was a pretty spot on parody, at least from what i saw on tv when staying home sick from school.

Mr Potato from Peppa Pig

black wings kaminari from rain webcomics

The Navidson Record from House of Leaves, although it's questionable whether it is supposed to actually exist within the narrative.