Entei is Molly Hale's father figure in Pokemon 3: the Movie, serving as the main antagonist (later anti-hero) of the film. He is created by the combined forces of Molly's dreams and the Unown's power.
When Molly Hale's father Prof. Spencer Hale finds several ruins pertaining to the existence of the Unown, he was sucked into their dimension. After Molly uses a box of tablets containing Unown images and begins assembling the letters, this summons the Unown, who use their powers to make Molly's wishes come true, transforming her manor house into a crystal-like palace which spreads across the town and cuts her off from the world. They also created a new Pokémon known as Entei to act as Molly's father.
As Molly wishes for a mother figure, Entei decides to make her wish come true by kidnapping Delia Ketchum and bringing her to Molly. He also uses his hypnotic powers to temporarily make Delia believe she is Molly's mother. This event caused Delia's son Ash (the protagonist of the series) to head out to the mansion and confront Entei to save her mother.
As Ash and his friends finally spot Delia and Molly, Ash angrily confronts Entei for kidnapping his mother, but Entei refuses to let Ash take Delia home and battles Ash's Pokémon. However, Molly (not wanting anyone to get hurt) convinces everyone to stop fighting, and Entei (realizing that he's been used by the Unown) reveals that he was created by the Unown, whose powers suddenly went out of control. As the threat of the Unown starts to endanger the world, Entei helps the heroes escape and sacrifices himself to help send the Unown back to the dimension. Entei's sacrifice also allowed Professor Hale to be released back from the Unown dimension and be reunited with Molly.