The predator / Applegate, Katherine

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.

Marco's not sure that having the power to morph is good - it's just leading him and his friends into danger. Wouldn't it be better to stay out of trouble? But something's about to change Marco's mind. He's about to discover a new horror, something that will give him a real reason to fight.

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.