Muriel and Floyd are the tertiary antagonists of The Secret of NIMH 2: Timmy to the Rescue. They are two stray cats who intend to eat the mice of NIMH, particularly Timothy Brisby.
Only one of them actually belongs in the "Singing Villains" category, and that would be Muriel.
Muriel and Floyd originally worked for Martin Brisby who has turned evil. Their final defeat is when Timothy races them to the elevator while its car is still out of sight, making them fall to the bottom of the building.