- "This tongue's sense of taste was beyond anything a human tongue could ever achieve. This tongue tasted the very air. And then, I felt the feature I had waited for. 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."
- "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.
- "What better way to strike suddenly than with the body of a snake?"
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. 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, and again in The Weakness to eliminate the Garatron. During the events of The Arrival, Marco and Cassie both use their snake morphs as they are dangerous yet capable of infiltration. Tom's Yeerk used a cobra morph as one of his battle morphs, while Jake and rebel Taxxons morphed anacondas in The Answer and The Beginning respectively, with the Taxxons deciding to trap themselves in the morph.
- 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)
- The Weakness (king cobra, morphed by Marco)
- The Arrival (pit viper, acquired and morphed by Cassie/king cobra, morphed by Marco)
- The Ultimate (rattlesnake, acquired and morphed by an Auxiliary Animorph)
- The Answer (cobra, acquired and morphed by Tom's Yeerk/anaconda, acquired and morphed by Jake; cover morph)
- The Beginning (anaconda, acquired and morphed by Taxxons, who become anaconda nothlits)