- "They're herd animals. Not very smart. In fact, pretty dumb, really. They can run fast, but there are lots of faster animals. They're strong, but there are lots of other animals that are stronger."
A horse is a four-legged equestrian native to Earth. It is notable for being Cassie's first morph and a morph that all six core Animorphs possess. The Yeerks also briefly infested horses in order to infiltrate Zone 91.
- "So I pressed both my hands against the heaving flank of the big horse and I focused my mind. He grew calm and quiet. His DNA flowed into me. It became a part of me."
Cassie acquired one of the horses on her farm in The Invasion, her very first acquisition as well as her very first morph. A few months later, in The Unknown, the six Animorphs acquire racehorses at a racetrack, with Cassie acquiring derby-winning golden racehorse Minneapolis Max. Jake also acquires a war horse in Elfangor's Secret, when the Animorphs traveled back in time to Azincourt, France in 1415.
Cassie used her horse morph to test out the morphing ability in The Invasion, and as her only available morph, used it again during the Animorphs' first attack on the Yeerk pool, saving a woman from infestation and presumably dealing with the police officer-Controller who had suspected her. Shortly thereafter, following an attack on a Yeerk-run hospital where Jake was infested by Temrash 114, Marco told Cassie to use her horse morph to transport Jake to safety. Cassie also had a habit of using her horse morph to run across the fields of her farm at night, although she wished to keep it a secret from Jake, as he frowned upon using morphing for personal reasons.
After discovering that the Yeerks were infesting horses in order to infiltrate Zone 91, the Animorphs decided to use horse morphs to enter the base as well, using racehorse morphs they had acquired at a racetrack. The horse morph would be used one last time in Elfangor's Secret, with Cassie and Jake morphing horses, the latter a 1415 war horse, after the Animorphs followed Visser Four back in time.
- The Invasion (acquired and morphed by Cassie)
- The Visitor (mentioned)
- The Encounter (mentioned)
- The Message (mentioned)
- The Predator (mentioned)
- The Capture (morphed by Cassie)
- The Alien (morphed by Cassie)
- The Secret
- The Reaction (mentioned)
- The Andalite Chronicles (mentioned)
- The Unknown (Horse-Controllers/wild horses/racehorses; acquired and morphed by Jake, Rachel, Tobias, Cassie, Marco and Ax)
- The Warning (mentioned)
- The Departure (Cassie's mare)
- The Threat (mentioned)
- The Suspicion (mentioned)
- The Extreme (mentioned)
- Elfangor's Secret (horse, morphed by Cassie/war horse; acquired and morphed by Jake)
- The Reunion (mentioned)
- VISSER (mentioned)
- The Hidden (mentioned)
- The Other (mentioned)
- Back to Before (mentioned)
- The Familiar (mentioned)
- The Test (mentioned)
- The Resistance
- The Diversion (mentioned)
- The Sacrifice (mentioned)
- The Answer (mentioned)