An R-rated sci-fi/horror classic, The Toxic Avenger got a cartoon.
Said cartoons met with varying degrees of success and aren't nearly as common now, but it's not hard to understand why studios go this route. To kids, the latest R-rated franchise might seem like the epitome of cool, and something they aren't supposed to watch. By creating a cartoon, those younger viewers can get a taste of those franchises' worlds without being subjected to the R-rated content. This can even be true for video games, with Mortal Kombat's kid-friendly Defenders of the Realm cartoon.
In the 1980s, Alien and Aliens were both regarded as excellent films, and both were big hits. Aliens took things into a more action-heavy direction and introduced space marines and pulse rifles, priming it for merchandising and toy opportunities as well as its own animated tie-in. Oddly, such a show wasn't actually pitched to Fox until two decades later in the mid-2000s. Sadly, Aliens: War Games never made it past the development stage, denying fans a cartoon bug hunt.
Aliens: War Games - The Cartoon That Never Happened
In 2007, a company called Bluefields Creative developed and pitch an idea for a cartoon show called Aliens: War Games. The plot centered on Newbie, a new recruit to the Colonial Marines in a future where the threat of Xenomorphs is no longer under wraps, and they're considered a clear and present danger to humanity. The first season would've focused on Newbie's ever crazier training exercises, which would involve him squaring off with captive Xenomorphs in a semi-controlled environment. By the end of the season, he would've graduated and been sent to the frontlines of the war.
Aliens: War Games was planned to employ a similar animation style to Cartoon Network's Prometheus.