Humpback whales, also known as the great one(s), are sentient lifeforms  who can communicate among both themselves and other sea creatures, such as bottlenose dolphins, using a form of telepathy. The Animorphs first encountered one in The Message, where it told them where the Andalite Dome ship was after the Animorphs rescued it from sharks. The whale also helped save Marco as gratitude. Later on, the whale and its allies save the Animorphs from being killed by Visser Three in his Mardrut morph, and tows the Animorphs and Ax back to shore. In The Andalite's Gift, Tobias locates a humpback whale as a request from Cassie, who acquired and morphed the whale in order to kill the Veleek, a morph-hunter.
Sperm whales first appeared in The Message, where a pair helped a humpback whale save the Animorphs from Visser Three. In The Exposed, the Drode causes a sperm whale to beach in order to give the Animorphs a chance to acquire it; Tobias and Rachel acquire and morph the sperm whale in order to catch a giant squid.
David acquires and morphs an orca in The Solution, in an attempt to kill the Animorphs while they are in their dolphin morphs. In The Mutation, all six of the Animorphs, including Ax, acquire and morph Swoosh, an orca from The Gardens, for a deep sea exploration mission.
- The Message (humpback whales, sperm whales)
- The Andalite's Gift (humpback whale; acquired and morphed by Cassie)
- The Change (humpback whale; mentioned)
- The Escape (mentioned)
- The Warning (mentioned)
- In the Time of Dinosaurs (mentioned)
- The Solution (orca; acquired and morphed by David/humpback whale, morphed by Cassie)
- The Suspicion (humpback whale, morphed by Cassie)
- The Extreme (orca)
- The Exposed (sperm whale; acquired and morphed by Rachel and Tobias)
- The Mutation (orca; acquired and morphed by Jake, Rachel, Tobias, Cassie, Marco and Ax)