There's a scene in Animorphs where 'Ax', roughly a centaur body-form alien, crash landed on Earth...

Either one or two of the main characters find him, hide him in some cabin or something, and Ax begins to freak out, beg his human friends to stop harming themselves...

When they change into clothes that aren't waterlogged and covered in mud.

Because he has no concept of 'clothing', and thinks they are ripping their own bodies apart.

EDIT:

Derp, I am a dumbass and blended together two different scenes and characters.

It's not Ax who roughly has the above described seen play out.

It is Elfangor, a different Andalite, Aximili's older brother, and IIRC, that actual story about the clothes takes place outside of the mainline books, happens in one of the... couple of sort of extra canon books, that follow different characters, not directly in sync with the mainline serial books.