The Family Guy team regularly throws in a lot of references to film and television and their effort on ThunderCats was masterful. The scene appears in part two of Family Guy Presents Stewie Griffin: The Untold Story (Oct 2005). This is sometimes credited as season 4 episode 29 though you won’t find it on a DVD collection for the season – it was released as a standalone movie. When the movie was repackaged and aired during season four the scene was cut, so you can only find it on the DVD movie version.
Both Larry Kenney and Lynne Lipton reprised their voice acting roles for Lion-O and Cheetara respectively. Kudos to Seth McFarlane and the Familiy Guy team for bringing this talented pair in, it elevates the scene greatly.
I won’t give away what happens, just enjoy the clip.