Coolidge, currently on screen in the ensemble comedy For Your Consideration, will play the village vixen Sybil, while Leno will voice Malaria's conniving ruler King Malbert.
The Weinstein Company holds worldwide rights to Igor, which tells of a hunchbacked laboratory assistant who dreams of becoming a famous scientist. Steve Buscemi, Jeremy Piven, Molly Shannon, John Cleese and Christian Slater have previously signed on.
'Jay and Jennifer each have unique voices and brilliant comedic timing, both of which are ideal for the film,' Igor director Tony Leondis said. 'We're thrilled to have them join an already stellar voice cast.'