Given that the horror genre is one that thrives off of imitation and the influence of what has come before, it's only natural that writers and filmmakers love to slip in a couple of cheeky references to their favourite scary movies. They either tend to go down the Scream route and have self-aware characters pay lip service to specific films or in some instances, take a more subtle approach.
As opposed to referencing a film's existence as simply a film, many filmmakers instead prefer to use visual cues to imply that there is a shared universe between their film and another pre-existing property. These types of references tend to send fans into a frenzy, and as a result, the horror genre has been awash with multiple easter-eggs that bind together many well-loved movies.
In regards to the connections themselves, this isn't a simple case of the films sharing the same writer or director. Instead, it needs to be a little deeper than that, with an overt reference or something that implies that both films take place within the same universe. Be it a shared setting, a cameo appearance by a minor character, or simply a line of dialogue that pertains to another film's events.
Not all of the inclusions featured here today are considered canon in terms of being shared universes, but the connective elements that bind these films together make a clear enough distinction that on a surface level both films take place within the same world.
10. Planet Terror & From Dusk Till Dawn
If you're ever in the mood for one hell of a double-bill, I can't recommend Planet Terror and From Dusk Till Dawn enough. Both of them are campy, gore-filled, action-heavy extravaganzas, that are full of quippy dialogue and fantastic characters.
They also just so happen to exist within the same universe.
Aside from the fact that both films are helmed by the always over-the-top Robert Rodriguez, it is the appearance of Earl McGraw, that ties these two B-Movie spectaculars together. Earl is a Texas Sheriff who constantly becomes embroiled in strange goings-on. McGraw first appears in From Dusk Till Dawn, running afoul of the Gecko Brothers who swiftly fill him full of lead.
Somehow McGraw survives this and pops up again in Planet Terror, only here do we see a much softer side to his character, as he cares for his terminally ill wife. Although things quickly turn sour when his beloved wife turns into a zombie, and he has to plant an axe in her skull.
Earl also makes a small appearance in Kill Bill Vol.1, arriving at the scene of the wedding rehearsal massacre only to get a face-full of gob courtesy of a comatose Uma Thurman.