The Predator (Animorphs, No. 5)

Marco is a reluctant Animorph, having been given the ability, along with four other friends, to change into animal forms by an Andalite prince, an alien who wants help in stopping an invasion of the enemy Yeerks, but when he discovers that his mom, who he believed was dead, has been taken over by the Yeerks, he finds a reason to fight.