- "I held onto that stupid snake and I focused. When animals are acquired, which is when we absorb their DNA, they become calm, relaxed, peaceful. Most of the time, anyway. But not the snake. No, as I absorbed the DNA and as David's father went to the closet, that lunatic snake kept thrashing like an idiot."
- ―Marco acquiring Spawn
A rattlesnake was first acquired by Ax in The Alien, who acquired it as he had less Earth animal morphs compared to the other Animorphs. Visser Three did acquire a snake native to the Amazon rainforest in The Forgotten, but the timeline was erased due to the effects of the Sario Rip. Marco acquired Spawn, a king cobra owned by David, in The Discovery while David acquired a rattlesnake in The Solution. Cassie acquired a pit viper in The Arrival, while Jake and a colony of rebel Taxxons acquired and morphed anacondas in The Answer; Tom's Yeerk also possesses a cobra morph. In addition to the main Animorphs, one of the Auxiliary Animorphs acquired a rattlesnake to use as a battle morph in The Ultimate.
- "The rattlesnake. That would be the morph to use. What better way to strike suddenly than with the body of a snake? [...] Huge, long, curved fangs. Fangs that were each a tiny, hollow needle. Above them poison glands grew and filled with toxin. I felt the snake's mind emerge beneath my own awareness. It was not a hot, driven mind like some animals. It did not overwhelm me with fear and hunger. It was a slow, calm, deliberate mind. The mind of a predator. A hunter. A calm, deliberate killer."
Ax first uses his rattlesnake morph in The Alien, attempting to kill Visser Three without the Visser noticing him. While he does bite Visser Three's Andalite body, the Yeerk escapes, allowing Ax a moment to speak with the host, Alloran-Semitur-Corrass. Ax later uses the rattlesnake morph in The Unknown, while fighting the Yeerks at The Gardens, due to the fact that there are many humans in the vicinity and thus cannot be in his Andalite form. David used a rattlesnake morph to spy on the Animorphs in The Solution, later using the morph to constrict Rachel in rat morph and threaten to kill her unless the other Animorphs did not morph into cockroaches. Cassie used a pit viper morph, possibly a rattlesnake, in The Arrival, while one of the Auxiliary Animorphs used a rattlesnake as their battle morph.
- "Rachel was carrying Marco. And Marco was a snake. In fact, he was the cobra David used to own. David's snake had been made safe by the removal of its poison sacs. But since Marco morphed from the DNA, the surgery was irrelevant. Marco was a fully functional cobra, with venom that could knock a horse down in seconds and kill it in minutes."
Marco morphs into Spawn, David's pet king cobra, in The Discovery as it is the only animal that could be in David's room and not be found suspicious by David's father. Due to the arrival of Visser Three and his Yeerk forces, Marco was forced to use the cobra morph during the massive battle that ensues. He uses the morph again in The Threat, in order to bite humans and knock them out, in The Weakness to eliminate the Garatron, and once again in The Arrival as a countermeasure against Unit 0. Tom's Yeerk also possessed a cobra morph, which he used to fight Rachel in grizzly bear morph, although Rachel managed to impale the cobra with her claws and bite down on the snake, killing Tom and his Yeerk.
- The Predator (mentioned)
- The Alien (rattlesnake, acquired and morphed by Ax)
- The Forgotten (Amazonian snakes, one is acquired and used by Visser Three; rendered usable due to the Sario Rip)
- The Reaction (pythons)
- The Unknown (wild bull snake. rattlesnake; morphed by Ax)
- The Decision (Leeran snakes)
- In the Time of Dinosaurs (rattlesnakes; mentioned)
- The Discovery (king cobra, acquired and morphed by Marco/cover morph)
- The Threat (king cobra, morphed by Marco)
- The Solution (rattlesnake, acquired and morphed by David)
- The Hork-Bajir Chronicles (mentioned)
- The Pretender (mentioned)
- The Extreme (mentioned)
- The Exposed (mentioned)
- Elfangor's Secret (python; mentioned)
- The Proposal (mentioned)
- The Weakness (king cobra, morphed by Marco)
- The Arrival (pit viper, acquired and morphed by Cassie/king cobra, morphed by Marco)
- Back to Before (python; mentioned)
- The Test (mentioned)
- The Diversion (diamondback rattlesnake, black garden snake)
- The Ultimate (rattlesnake, acquired and morphed by an Auxiliary Animorph)
- The Sacrifice (mentioned)
- The Answer (anaconda, acquired and morphed by Jake/cover morph)
- The Beginning (cobra, morphed by Tom's Yeerk using Tom's body/anacondas and other snakes, acquired and morphed by Arbron's Taxxons, nothlits)
- Ax (rattlesnake)
- Esplin 9466 (Amazonian snake, using Alloran-Semitur-Corrass's body; morph unusable due to Sario Rip)
- Marco (king cobra)
- David (rattlesnake)
- Cassie (pit viper)
- Unnamed Auxiliary Animorph (rattlesnake)
- Jake (anaconda)
- Tom / Tom's Yeerk (cobra, using Tom's body)
- Taxxons (anacondas and other snakes; nothlit)