Argh. *breaks stuff, walks around staring at people in fury*
*breaks some more stuff*
They weren't original so much as effective. The series took a bunch from 50's sci-fi movies, mixed it with some 90's sarcastic pop-culture references, and threw it all into a plausible real-world setting.
You're right, I don't think the turning-into-animals idea had been used before, though, certainly not in a mainstream series.
Where Animorphs succeeded wasn't so much in originality though (despite the Andalites, and the morphing itself, etc, being pretty fresh), it was more about having the guts to not talk down to its audience.