How to move rent movies from iPod Touch to iMac?
According to the FAQ - you should be able to move a rental that was first purchased and downloaded to a Mac as many times as you wish - but it can only be on one device at any point in time. You need to be connected to the internet when you move the rental file.

In your case, just find the rental on the device in iTunes and transfer it back to the iTunes library.

Expand the section named - Can I play my rental on more than one device? for the exact wording by Apple on this.

If you rented it on iOS - It's not possible without cracking the DRM as rentals from iOS or AppleTV stay on that one device and cannot be transferred.