Horror is often the most shameless genre of cinema, so much so that it's a point of pride for many creators in the medium. There are theoretically endless ideas that could come to the mind of a creator when asked to scare an audience, but a few seem to draw repeat customers.
There is an enormous and absurd variety of types of media under the horror movie umbrella, including some of the most powerful and least worthwhile outings in film history. Entire industries are living in the walls of better production outlets and stealing whatever scraps they can repackage and resell for a quick buck.
R.O.T.O.R.
Paul Verhoeven's Robocop dropped in the summer of 1987 and immediately became one of the most heavily ripped-off films in cinematic history. In the late-80s and early-90s, regular people donned cheap cop costumes and walked with awkwardly locked joints to evoke cybernetics and called it a horror film. R.O.T.O.R. might be the funniest example of this concept because it's also a complete ripoff of James Cameron's Terminator.
The story follows police roboticist Dr. J. Barrett C. Coldyron who is in the process of crafting some sort of robot cop. The corrupt bureaucrats that sign Coldyron's checks demand results years in advance of what's possible, so he's forced to resign. As a result, the Robotic Officer of the Tactical Operations Research/Reserve Units set loose with the apparent mandate to execute anyone he sees commit a crime. Coldyron must stop his creation to stop it from mass murder. This film is hilarious when it isn't crushingly boring. The leather-clad take on the T-800 lacks any intimidation but tries his best. This one is worth a watch for the laughs.
Ghoulies
This film uniquely walks the line between ripoff and coincidence. Ghoulies and Gremlins were in production at the same time, Warner Bros. even sued the cheaper film for snagging the name. The critical response to this 1985 tiny monster movie couldn't go more than a sentence or two without mentioning Joe Dante's beloved comedy.
This film is packed with terrible fake gore, bad jokes, and gross practical effects. The eponymous beasts, small reptilian puppets who are always slathered in some sort of goo, originate from a Satanic ritual. Ghoulies actually made a substantial return at the box office, despite the immensely negative reviews. At 81 minutes, there's a certain 50s B-movie charm that may get many audiences through this film, but it's a pretty rough watch for those looking for anything substantial.
Orca
Yes, there are around a million shark movies out there, and not one of them would exist without ripping off Jaws, but Orca is a direct effort to best Spielberg's classic. Evil Dead II producer Dino de Laurentiis saw Jaws and demanded a movie about a bigger aquatic threat. What he got was Michael Anderson's 1977 epic Orca: The Killer Whale, which probably didn't make its budget back and definitely got savaged by critics.
The film focuses on Nolan, an Irish fisherman hunting a great white shark. While doing battle with the iconic fish, a killer whale slaughters it in combat, a bold statement of intent that the film spectacularly fails to achieve. Nolan and his fellow fisherman slay the orca's mate, leading it on a violent vengeance quest against the people of Nova Scotia. This film isn't as bad as some make it out to be but confidently seeking to dethrone the original summer blockbuster puts a bad taste in the mouth of most audiences.
Birdemic: Shock and Terror
James Nguyen's 2010 horror epic is in the camp of outwardly vocal ripoffs. The writer/director has been extremely clear that his killer birds film is a pastiche of Hitchcock's The Birds. Nguyen worked with Birds star Tippi Hedren in his film Julie and Jack, which can be seen on a TV screen in Birdemic. Birdemic sits comfortably among most lists of the best bad movies of all time, earning over a decade of thrilling laughter for its troubles.
The film features abysmal special effects, terrible acting, long stretches of pointless B-roll, a weirdly preachy environmental message, and some of the worst sounds ever committed to DVD. The two main characters are young lovers trying to evade the assault of mutated birds. It's not worth panning a film that exists as a laughingstock, it's far better to join in the laughter. Fun fact: both this film and R.O.T.O.R. are available on YouTube for free with RiffTrax commentary, and both are perfect bad movie night viewing.
Paranormal Entity
One could fill a thousand lists with ripoffs by simply looking up the output of The Asylum. Their "mockbusters" haunt various DVD bins and streaming services, hoping to trick a confused viewer with a similar title. Paranormal Activity is the most profitable horror film ever made, so of course, The Asylum would take a shot at that.
The film is a found-footage ghost film that sells itself as real videotape evidence in a murder trial. It's decent when compared to the average Asylum project, partially because Paranormal Activity costs very little to make. The Asylum is as shameless as it gets, one example barely sticks out above the rest.