Entei was a physical manifestation created by the Unown of Molly's desires and lust for control. It is an illusionary Entei that appeared to Molly Hale after she made a wish to the Unown. It served as a replacement father for her while her real father, Spencer, was trapped in the Unown Dimension. Entei was able to speak in the humans' language because it was an illusion created by the Unown.
Entei and Molly
It did everything Molly asked for, even kidnapping Delia so she could be Molly's mother and providing her with illusory Pokémon for her to battle Brock and Misty with when they entered the tower that Entei had created to rescue Delia.
When Ash attempted to confront Entei to get his mother back, his Totodile and Cyndaquil were rapidly defeated, with Pikachu only lasting longer without faring any better, and even Charizard—who had witnessed the incident on television at the Charicific Valley and come to help—was only able to hold Entei off without actually defeating it. However, when Entei was about to break Charizard's neck, Molly finally decided that she wanted things to be real again.
Accepting Molly's decision, Entei helped Pikachu and Charizard defeat the now uncontrollable Unown, subsequently bidding Molly farewell and assuring her that it would always be in her dreams.
Since Entei was created as a father for Molly by the Unown, it acted like one, giving Molly everything she wanted, though not real, for her to be happy, going so far as to steal Ash's mom to give Molly a mother. It is also very protective of Molly and her wishes, fighting Ash and his Pokémon to protect Molly and her "mama". However, despite its origin as an illusion, it still had a mind of its own, allowing it to accept Molly's decision to return to reality rather than live in the illusion it created, and even acting against the Unown who created it.
Although it was not, Entei acted very similar to Molly's real father, Spencer Hale, loving her with a real father's love, and even bade her farewell with the same words as Spencer, telling Molly "Just keep me close in your dreams" before it vanished.
Entei Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.