I hope this is an appropriate section for this topic. 1979's Alien was a game-changer as we all know. And it didn't take long for filmmakers to start churning out cash-ins inspired by the Alien formula.
James Cameron himself even worked on some of these projects before moving on to his success with The Terminator and of course, Aliens.
Here are some examples that I can think of right off the top of my head.
I'd really like to know of any more, other than the films I have listed here. I enjoy watching them.
Mutant (aka Forbidden World) - A fun movie. The creature, as you can see in this poster is very much like the one brought onto the Nostromo from LV-426. If you haven't seen it, it's available to watch free on YouTube -
https://www.youtube.com/watch?v=go1eQlpyAaIReturn of the Aliens: The Deadly Spawn - an all-time favorite of mine. This is not so much a 'rip-off' of Alien, but they did attempt to capitalize with the 'Return of the Aliens' nudge. Very good movie, though. I highly recommend it. It also has a sequel called 'Metamorphosis: The Alien Factor' which is very entertaining as well.
Promoted as Alien 2 in Europe, I believe.
Inseminoid - I haven't actually seen this one yet, but will.
https://www.youtube.com/watch?v=6OyqqWNwfKcI haven't seen this one yet, either. Just learned of it, actually. It is apparently an almost blatant rip-off of Aliens, but with a tinge of Terminator, and it was promoted in Europe as 'Terminator 2.' It's primary title now is
Shocking Dark. Also on YouTube. Gonna check it out soon:
https://www.youtube.com/watch?v=4Ca1N4hFFQk