With the last two major Zonks to appear in the latest episode of 'Supernatural', we're dealing with pop culture references that are just as multi-universal as Felix and Oscar (since today is their anniversary, t'hee).
Both allusions were made by Dean:
"You remember 'redrum'."
"You know, Casper the Bloodthirsty Ghost."
"redrum" is "murder" spelled backwards, and it featured prominently in a novel by Stephen King, "The Shining". That novel was adapted for other media twice. First, as a movie by Stanley Kubrick starring Jack Nicholson. And then as a TV mini-series starring Stephen Webber. (Perhaps preparing for his role as Jack Rudolph in 'Studio 60'?)
We can dismiss the book and the film, since they take place in other universes born of Mankind's creative imagination. But the televersion must be taken into account; that it must have actually taken place in Toobworld. The book was published in 1977, and the movie came out in the early 80s, but the TV mini-series was only broadcast just a few years back.
As such, the horrific events that transpired over that winter at the Overlook Hotel in Colorado must have made national headlines. Sam and Dean may have even looked into the matter over the summer during the season hiatus. And so of course the gathered Baltimore cops would recognize the phrase "redrum", as it would have been splattered all over the newspaper accounts all across the country.
As for "Caspar The Friendly Ghost", this spirit of a dead child (Let's face it - that's what he was!) was the subject of some 1940s cartoons and then Harvey comic books which are still being published today. And then there was a live-action movie that used the latest in CGI for its time, and which featured a cameo by a Toobworld character, Father Guido Sarducci, who first came to prominence on 'Saturday Night Live'. (But as was the case with Barnaby Jones in the movie about "The Beverly Hillbillies", Father Sarducci's appearance in this film was by the priest who exists in the Cineverse, not Toobworld.)
Caspar has had a few TV series based on his adventures, but those are set in the Tooniverse which - although it is a TV dimension, - is separate from that of Earth Prime-Time. The people in both dimensions are aware of the existence of the others, but for the most part they think of them as being fictional (never realizing that they all are.)
But there have been a few TV movies which feature the adventures of Caspar, in continuation from the 1995 movie:
"Casper: A Spirited Beginning" (1997) (TV)
"Casper Meets Wendy" (1998) (TV).
2000's "Casper's Haunted Christma"s and "Casper's Scare School" from 2005 are both animated features, so they can be relegated to the Tooniverse.
The other two are live-action, and although they continue on from the original theatrical release, neither one features the characters played by Bill Pullman and Christina Ricci. In "A Spirited Beginning", it's mostly flashback as Caspar narrates the story of his life. "Caspar Meets Wendy" tells of his first encounter with the little girl witch (played by Hilary Duff).
Dean would have known of Caspar's existence (or lack thereof). Even though the mood of 'Supernatural' clashes with that of 'Caspar', it's all one in the TV dimension. It's the same kind of mix that you get when 'Lou Grant' leaves 'The Mary Tyler Moore Show' for his eponymous drama series.
Whether or not the others knew of the young (relatively speaking) ghost is besides the point. Even though Dean phrased the reference as though he expected them to understand ("You know, Casper the Bloodthirsty Ghost."), in the end it didn't matter. No matter what name Dean gave, it's the description as a "Blood-Thirsty Ghost" that would have been all that mattered to get the point across. Well, I think that's about enough spooky goings-on for the day. There's always tomorrow for more fun and adventure in Toobworld.
That is.... if you wake up.......
Sleep tight, kids!
"Caspar The Friendly Ghost"