During one of their lessons, Merlin turns himself and the boy into squirrels and immediately attracts the attention of a female tree rat. The girl squirrel instantly falls in love with the animorphed Arthur, rubbing herself all over him and giving him one of the most explicit "Fuck me" eyes since the sultry duster from Beauty And The Beast. And I want you to know I had like ten solid "nut"-based jokes about this scene, but I'm keeping them to myself out of respect for what happens next.
After risking her life to save Squirrel Arthur from a wolf, the girl rodent hugs her beloved right as Merlin transforms him back to a boy. Arthur laughs the whole thing off, but the squirrel is in complete shock. She frantically looks Arthur up and down, not knowing what happened to the guy she was just with. One minute she was blissfully in love, and then out of nowhere, she's been thrust into a world of Cronenbergian horror. From her perspective, someone she fell in love with just disappeared and was replaced by this strange gigantic alien creature, and it completely messes with her head.
Confused and scared, she runs up a tree, hides in a hole and ... just starts crying and sniffling. She then glances at Arthur with a pained look in her eyes that seems to beg him to please, PLEASE give her her squirrel boy back. But of course, that never happens, and she is forced to somberly watch as Arthur and Merlin walk away from atop a dead tree, which here symbolizes all the hope the viewers no longer have for humanity. That's the last we ever see of her. That little squirrel's life is ruined, and so is her entire concept of reality. It's difficult to come back from "The love of my squirrel life turned into a non-squirrel monster without warning and then abandoned me forever."
The scene messed up so many Disney fans that an entire fandom movement grew around the idea that Merlin eventually went back and turned the squirrel into a human. Her name is now Hazel, and she gets to go on a lot of cool adventures, many (but not all) of which are surprisingly not porn.