The name of the Animorph book series, by K. A. Applegate. Five teenagers are granted powers to 'morph' or become other animals by a dying alien Andalite prince named Elfangor, killed by his enemy, Visser Three, an intelligent parasite called a Yeerk inhabiting another Andalite's body.
The teenagers, Jake, Rachel, Tobias, Cassie, and Marco, use their new powers to fight the spreading Yeerk infection on earth. They later teem up with Elfangor's brother, Aximili.
The book series encompasses 54 books, 4 Megamorphs, 4 other books detailing the history of different people/races mentioned, and several other Alternamorphs, which are choose-your-own-ending books.
I am an Animorph.
Would you like to become an Animorph?
The Animorph's are cool.