Animorphs

Animorphs was a book series created by Katherine Applegate and published by Scholastic. There were 64 books in all, including 54 'main universe' books, four 'megamorphs' (double-sized books), four standalone novels, and two choose-your-own-adventure. In addition, there were Animorphs Transformers, video games, and a short-lived TV show on Nickelodeon.

In the series, the main enemy was the Yeerks, an alien species of mind-controlling parasite. In order to fight the yeerks, a group of five young humans is given the gift of shapeshifting by a dying alien.

The shapeshifting ability allows these humans, who adopt the name Animorphs for themselves, to absorb DNA or genetic material by touch, and then transform into that animal. They can only remain shapeshifted for a maximum of two hours, however; after this the change is permanent. This fate befalls the main character Tobias in the first book, and his difficulties with being stuck as an animal are explored over the series.

The six main characters are Jake (signature morph: tiger), Cassie (signature morph: wolf), Marco (signature morph:  gorilla), Rachel (signature morph:  elephant until book seven at which point it alternates between elephant and grizzly bear), Tobias (signature morph:  red-tailed hawk), and Aximili (an Andalite, a sort of psychic deer-scorpion taur). Aximili was rescued from the remnants of an Andalite Dome Ship in the fourth book. In the next (fifth) book, Marco discovers that his mother ((sometimes called Eva, other times called Laura)) is Visser One. In book six, Jake accidentally falls into a Yeerk Pool and ends up with the Yeerk, named Temrash, who used to be in his brother Tom's head in his. At the very end, Jake sees a Big Red Eye, who isn't named until the twenty-sixth book. The fanbase at this point was already very strong, and was able to persuade Applegate to bring back the B.R.E., as the fans had started calling him. Another fan-induced idea happened in books five and thirteen, when a member of the usually dumb Hork-Bajir "started talking like he had a Harvard education", as Marco put it in book five. Eight books later, a name was given to this type of Hork-Bajir: Seer. A fan named Erin K. Dover had persuaded Applegate to use intelligent Hork-Bajir. ((Note: The original term Dover used was Hork-Bajir Thinker, not Hork-Bajir Seer.))

In the twenty-third book, it is discovered that Aximili, or Ax, as the others call him, is Tobias' uncle, although the fans had known this ever since the Andalite Chronicles came out.