I am writing an essay for school about "The Neighborhood of the Future" And I want to include the babel fish from a Hitchhiker's Guide to the Galaxy. Could this be considered plagiarism?
It's not plagiarism if you make it clear that you are referring to someone else's idea. Don't pretend it was your idea; there's a good chance your teacher has read that book and will recognise it. You should say where the idea came from. There are standard referencing styles for doing this which you may use. You can find examples online.