Animorph90's Animorphs
The Plot
Just in case you have not read Animorphs, here is what it is about. There are aliens that have invaded the planet earth, in secret. They are called Yeerks. They are these large slugs that can crawl into someone's ear canal and take over their brain, and once they do that they are able to control that person. This is scary, because it means that if they take over someone, that person still looks human and can blend into society, and so you cannot tell that they are being controlled by an alien.
The Animorphs are a group of teenagers that work together to try to defeat the Yeerks. They have a special ability which helps them do that. They are able to transform into any animal that they can touch. This is useful, especially when they transform into an animal that is very small, or has the ability to fly. Unfortunately, once they transform into an animal, they only have two hours to switch back to human form, otherwise they will be trapped in animal form. The Animorphs have to be careful to avoid this.
There are also good aliens, called Andalites. They have advanced technology, and they are the ones that gave the Animorphs their ability to morph into animals. The Andalites do not look human. They are centaurs and have four legs. The Animorphs actually have an Andalite included in their group named Ax. I do not remember how Ax is able to successfully live in human society while hiding that he is an alien. I think it might be that he always just morphs into a human whenever he is in public.