There's a difference between getting inspired by, and a total copy or a rip off. It's a complicated, vague topic. What is different is that zombies, vampires and werewolves are the result of old folklore and stories from way back, that became legends over time. Space Invaders is a single IP, but aliens and ufo's are not. Also after some time there are no more rights, for example, using images of old paintings is totally fine. But "recent" work can not simply be copied, you'd need a license to use the property.
But yeah, it has been done a lot if we're talking about Space Invaders, I think because it has been around for a while and a revolutionary game. Like loic said, as long as you use other assets, another name and don't use someone else's IP to make profit.. I guess you're fine. But when you look at Candy Crush it's a different story.