Cat People

Jane Randolph in the trailer for Cat People
At the Central Park Zoo in New York City, Serbian-born fashion designer Irena Dubrovna makes sketches of a black panther. She catches the attention of marine engineer Oliver Reed, who strikes up a conversation. Irena invites him to her apartment for tea. As they walk away, one of Irena's discarded sketches is revealed as a panther impaled by a sword.
At her apartment, Oliver is intrigued by a statue of a medieval warrior on horseback impaling a large cat with his sword. Irena informs Oliver that the figure is King John of Serbia and that the cat represents evil. According to legend, long ago the Christian residents of her home village gradually turned to witchcraft and devil worship after being enslaved by the Mameluks. When King John drove the Mameluks out and saw what the villagers had become, he had them killed. However, "the wisest and the most wicked" escaped into the mountains.
Oliver buys her a kitten, but upon meeting her it hisses. Irena explains that "cats just don't like me" and suggests they go to the pet shop to exchange it, but when they enter the shop the animals go wild in her presence. The shopkeeper says that animals can sense things about people. It gradually becomes clear that Irena believes she is descended from the cat people of her village, and that she fears that she will transform into a panther if aroused to passion.
Despite her odd beliefs, Oliver persuades her to marry him. However, during the dinner after their wedding at a Serbian restaurant, a catlike woman walks over and asks Irena if she is "Ð¼Ð¾Ña ÑÐµÑÑÑa" (moya sestra, "my sister"). Fearing something evil within her and dreading what could happen, Irena avoids sleeping with her husband. He persuades her to see a psychiatrist, Dr. Louis Judd. Irena tells Judd of her childhood, revealing that the other children had called her mother a cat person, and her father had died mysteriously. Judd tries to convince her that her fears are of a mundane nature, and stem from these childhood traumas. When Irena returns from her consultation with Judd, she discovers that Oliver has confided their marital problems to his attractive assistant, Alice Moore, and she feels betrayed.
At work, Alice confesses to Oliver that she loves him. After Irena chances to see Oliver and Alice seated together at a restaurant, she follows Alice as she walks home alone through one of the Central Park transverses. Alice becomes increasingly uneasy, sensing an unseen someone or something behind her. Just as she hears a menacing sound, a bus pulls up, and she hastily boards it. Soon after, a groundskeeper discovers several freshly killed sheep. The bloodied pawprints leading away turn into imprints of a woman's shoes.
Later, when Alice decides to take a dip in the basement swimming pool of her apartment building, she is stalked by an animal shown only by its shadow. She jumps into the pool, using the water to keep the creature at bay. When Alice screams for help, Irena turns on the lights and claims to be looking for Oliver. Alice emerges, wondering if she had imagined the whole thing, until she finds her robe torn to shreds.
After a talk with Judd, Irena tells Oliver she is no longer afraid, but it is too late; Oliver has realized that he loves Alice and is getting a divorce. Later, at work, he and Alice are cornered by a ferocious animal. Thinking quickly, he grabs his T-square (which is in the shape of a cross) and tells Irena to go away.
After it leaves, Alice calls Judd to warn him to stay away from Irena, but he hangs up when Irena arrives for her appointment with him. Attracted to her, he makes the fatal mistake of kissing her. She transforms into a panther and kills him, though he manages to wound her in the shoulder with the sword concealed in his cane. Oliver and Alice arrive a few minutes too late. Irena slips away, back in her human shape, and goes to the zoo. There, she opens the panther's cage and allows herself to be killed.

Question: Who stalked Alice?
Answer: An animal