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.
Her speaking voice was provided by Virginia Madsen, and her vocal effects by Frank Welker.
She is a mountain lion who lived in the woods and she wanted to eat Stuart and Snowbell.
Towards the beginning of the film, Reeko showed up without any tribute and bought his way out with a promise of bringing double his quota next time. When the time came and he had nothing to show for it, he conned Snowbell into going to fill his debt. When the Beast noticed Snowbell's thick, luxurious fur, she decided to make a rug out of him so she'd have something warm to lie on during the winter. She only refrained from eating him when he suggested she fatten him up first to have a bigger rug.
When Stuart learned that the Beast had Snowbell prisoner, he set out to rescue him. Later, with the help of Reeko and the other forest animals, he succeeded in trapping her, whereupon she was sent to the zoo. She was likely ill-received there, as the chopper pilot who took her noted that they would have to tranquilize her and shave off her fur to get rid of Reeko's spray.
At the end she got taken in the helicopter.