Eeyore is a pretty popular name to use in this context.
Eeyore is a character in the Winnie-the-Pooh books by A. A. Milne. He
is generally characterized as a pessimistic, gloomy, depressed,
anhedonic, old grey stuffed donkey who is a friend of the title
TVTropes names its depressed character trope "The Eeyore", and ...
It's not quite the opposite of Pollyanna but you might want to look at Cassandra from Greek mythology. See: https://en.m.wikipedia.org/wiki/Cassandra_(metaphor)
Cassandra was cursed with the gift of prophecy (generally of the doom and gloom pessimistic variety) but she was also cursed so that no one would take her seriously.
The phrase "Cassandra complex" ...
A pessimistic person, who always says that bad things are going to
happen, is sometimes called a Jeremiah.
In the Jewish and Christian religions, Jeremiah is recognised as being a 6th Century BCE
Hebrew prophet who said that Jerusalem would be defeated and that God
would become angry with the Jews and punish them. The Book of Jeremiah
in the ...
A "Gloomy Gus" works as a particularly pessimistic, unhappy, depressed or, well, gloomy person.
It originates from the early 20th century Happy Hooligan cartoons.
There is Dismal Jimmy from British slang but it is not common like Pollyanna.
MW defines as:
a man noted for depressing pessimistic predictions and frame of mind
Apparently, it was a nickname for King James II:
Influenced in his decision by his wife, Anne Hyde, who had previously converted to Catholicism, James, "Dismal Jimmy" as Nell Gwynne was ...
Was expanding my comment to @ptomato's answer but didn't get it committed within the 5-minute mark, so am elevating this to a full answer.
In books, "foreword" (not to be confused with "forward") and "introduction" are also common. However:
In "appendix", the "ap-" seems to just convey "on" or "with"; it's the "pend" that conveys "hanging", thus "downward" ...