During this novel, both the Yeerks and the Animorphs want to know what alien object the US government is hiding in Zone 91. The Yeerks therefore take over horses, and the Animorphs morph horses, and they all stampede into Zone 91 and see the object. The Yeerks don't know what it is, but Ax later IDs it as an Andalite space toilet!
However, there's a good chance that Visser Three might figure out that the object is an Andalite space toilet, once the Yeerks describe it to him! After all, he's in the body of an Andalite warrior named Alloran, who WOULD know what an Andalite space toilet looks like!
Great moments: Cassie winning a race in her racehorse morph.