In the first Animorphs book The Invasion, the group decide to attack the Yeerk Pool in order to save Jake's brother Tom. In order to achieve this, they all acquire animal DNA that would be suitable for an attack (gorilla, elephant etc.).
However, Cassie gets taken by one of the Controllers before their attack, so they go to rescue her. They are supposedly ready to do the assault before she is kidnapped, yet in later books it is never addressed which form she had taken in order to attack the Yeerk Pool.
Had she acquired a form that was used in a later book in the series, or was she planning on acquiring another form later? As far as I remember her only DNA acquisition at this point was a horse from her farm. Was she planning on morphing into a horse for the attack?