- "Dolphins are very intelligent. Not our kind of intelligence, but still, I guess they're one of the two or three smartest animals around."
The bottlenose dolphins are the Animorphs' main aquatic morph.
- "I pushed away my doubts, closed my eyes, and concentrated on the dolphin. She became peaceful and calm, as animals always do during the acquiring process."
- ―Cassie acquiring Monica
The Animorphs first acquire the dolphin morph in The Message, after initially deciding not to acquire them in The Invasion. They do this by going to The Gardens during a dolphin show. The dolphin trainer, Eileen lets Jake, Rachel, Cassie, and Marco pet the dolphins, named after the cast of Friends. This allows them to acquire the dolphins, with Cassie acquiring Monica. Tobias and Ax acquire their dolphin morphs in The Escape and In the Time of Dinosaurs respectively.
- "I love being a dolphin. How can you not love it? I'm not crazy about morphing insects. Especially the mindless little automatons like termites and ants. But I'm convinced that dolphins have souls. Or maybe it's just some arrangement of DNA-based characteristics that make them seem that way to humans. But whatever it is, whether it's something mystical or something real, I like it."
The dolphins are the Animorphs' main aquatic morph. They are used in most of the books having to do with the ocean. All of the six Animorphs have this morph. They are more used towards the beginning of the series, until they are replaced by whale or shark morphs. This is one of Cassie's favorite morphs. Ax uses his dolphin morph the least out of everyone, as he opts to use his shark or whale morphs instead.
- The Invasion (mentioned)
- The Message (Ross, Rachel, Chandler, Joey and Monica; acquired and morphed by Cassie, Jake, Rachel and Marco/cover morph)
- The Predator (mentioned)
- The Capture (mentioned)
- The Andalite's Gift (morphed by Jake, Rachel, Cassie and Marco)
- The Android (mentioned)
- The Reaction (morphed by Jake, Cassie and Marco)
- The Escape (acquired by Tobias; morphed by Jake, Rachel, Tobias, Cassie and Marco)
- In the Time of Dinosaurs (acquired by Ax; morphed by Jake, Rachel, Tobias, Cassie, Marco and Ax)
- The Departure (mentioned)
- The Discovery (mentioned)
- The Solution (morphed by Jake, Rachel, Tobias, Cassie, Marco and Ax)
- The Extreme (morphed by Cassie and Marco)
- The Attack (mentioned)
- The Exposed
- Elfangor's Secret
- The Conspiracy
- The Hidden
- The Beginning