Animorphs

The Animorphs was a popular children's series ostensibly written by KA Applegate (but mostly ghostwritten) during the late 90s and early 00s that focused on a group of young teenagers fighting a secret war against aliens who are trying to take over the Earth.

To put a twist on the theme, the main characters could turn into any animal for two hours at a time after they absorbed said animal's DNA.

And the aliens taking over were basically brain slugs.

A lot of fanfiction for the Animorphs these days tends to be from older teenagers who still remember the series fondly, but occasionally there are fics from younger fans who have discovered the series only recently.