For the 20th anniversary of Animorphs -- the series of YA books about kids with the power to shape-change who are fighting against a secret invasion of Earth -- author K.A. Applegate gave a retrospective interview to Entertainment Weekly, revealing the vast number of influences on the series. An explicit reference, though, came in the form of a name taken from Tolkien.
Image: Animorphs, Scholastic
In Animorphs, the Yeerks are slug-like beings that take over hosts by entering the brain through the ear. This makes them one in a long list of aliens that take over others against their will, and another in the long list of slug-shaped aliens. (I bet the Yeerks and the Goa'uld from Stargate got along great.) Applegate said, "The Yeerks were straight out of sci-fi standards like Invasion of the Body Snatchers and Star Trek, although the name is a wink-wink to Lord of the Rings, as the Elvish word for 'orc' is basically 'yeerk' with a different spelling."
The Elvish word she's referring to is "yrch", which is actually the plural for orc. And I always assumed the "y" was pronounced as a vowel and not the consonant y-sound. But if you combined them, you would absolutely get something like "yeerk". That's a quality shout-out they have made here.
One Yeerk villain wasn't influenced by Sauron or any other Tolkien character, but by Star Wars. Said Applegate:
The [major] villain of modern times is of course Darth Vader. He was cool, mysterious, evil, but with some charm and a lot of charisma. And over time we learned something about his motives. We tried to do that with Visser Three. Emotionless, relentless killing machines are ultimately boring as villains; you want some nuance and something just a bit attractive. You want the reader to feel the seduction of evil -- there's no virtue without temptation.
And while tapping into Darth Vader makes sure Animorphs has a strong science fiction pedigree, ultimately the real model for the series wasn't science fiction at all, but a World War II story.
"One of our models was the old TV series Combat! which followed a single squad around WWII. We wanted that same tight-knit feel, that same grunt's-eye-view," said Applegate.
So there you go. Put a war show, Star Wars, Star Trek, Invasion of the Bodysnatchers and Middle-Earth in a blender, add some shape-changing and the weirdest creature you can think of (no, really: Applegate says that the Andalites were meant to be the least familiar-looking creatures they could come up with), and you have Animorphs.