The Beast is the main antagonist of Stuart Little 3: Call of the Wild. She is a cougar (although her sounds are a mixture of those of Panthera cats, including tiger, leopard, lion, and jaguar instead of cougar) who threatened to eat the animals if they didn't bring her offerings of food.
She is voiced by Virginia Madsen.
She is a mountain lion who lives in the woods and she wants to eat Stuart and Snowbell.
Towards the beginning of the film, Reeko shows up without any tribute and brings his way out with a promise of bringing double his quota next time. When the time came and he has nothing to show for it, he cons Snowbell into going to fill his debt. When the Beast notices Snowbell's thick, luxurious fur, she decides to make a rug out of him so she'd have something warm to lie on during the winter. She only refrains from eating him when he suggests she'd fatten him up first to have a bigger rug.
When Stuart learns that the Beast has Snowbell prisoner, he sets out to rescue him. Later, with the help of Reeko and the other forest animals, he succeeds in trapping her, whereupon she is sent to the zoo. She is likely ill-received there, as the chopper pilot who takes her notes that they would have to tranquilize her and shave off her fur to get rid of Reeko's spray.
At the end, she gets taken in the helicopter.