I always wondered what would have happened if the Animorphs had trapped David as something else. Obviously nothing too powerful, or anything that would be able to walk into the Yeerk pool and say: "Yo. I gots some information for ya'll."
Feel free to include pictures of what you think they should have turned him into--especially if some of us have never heard of them before.
Feel free to think up creatures better or worse than his rat form.
If they had access to the Mariana's Trench (not the band, obviously), I'd have them acquire, morph, and dump him there as this:
http://24.media.tumblr.com/tumblr_m5xbgm68Jk1qzruoyo1_400.jpg Not only would he be far, far away from humans, and prey to much bigger fish, but he'd go mad in the deepest, darkest depths of the ocean. Suddenly the rat doesn't seem so awful.