Today, Giancarlo Volpe (Showrunner for "Green Lantern: The Animated Series") announced that Sinestro will Debut on the show in the episode "Prisoner of Sinestro" (which airs next Saturday). In the comics, Sinestro was the Green Lantern who taught Hal Jordan to be Green Lantern and used to be partners with Abin Sur (Hal's predecessor), however he lost the job for using the Green Lantern powers to instill fear. After being banished to an anti-matter universe where the weaponers there created a ring for him made of yellow energy, which after returning to his universe of origin, helped him become the Green Lantern Corps greatest enemy.
Giancarlo Volpe also reveals who will play him in an interview with Newsarama: "So seeing that gave us permission. It was great to finally get to put him in, because it doesn't feel quite like the Green Lantern universe unless he's in it. And I suppose I can tell you, we got Ron Perlman to play Sinestro! So, he lends a very interesting take and perspective on Sinestro's character. Plus that's a cool sci-fi fan gem there."