|"I have been living at these rocks for a thousand moons. Spare my life, and I shall grant you one wish."|
|— Mouthclaw asking Sunpelt to spare her life in Secrets of the Clans, page 163|
In Secrets of the Clans, Mouthclaw lived in a dark cave near Snakerocks in LionClan territory at the time of the ancient Clans, LionClan, LeopardClan, and TigerClan. She was the only snake in the entire forest. She had killed many great warriors from all the Clans and was feared by every cat. With her fangs, she could swallow a living cat whole, and she spits out deadly venom. All cats were forbidden to go to Snakerocks, as the leaders were afraid to lose more of their warriors.
Sunpelt, a warrior of LionClan, thinks he can defeat Mouthclaw and prove what a brave warrior he is, so he plans to kill her, so he can earn the forest's respect. One morning he waits and calls for Mouthclaw outside of her cave, and starts kicking stones into her cave. She eventually comes out, angry. She wants to fight Sunpelt, and kill him, because he made her angry. LionClan warriors are one of her favourite meals, so she attacks him, but Sunpelt dodges. Mouthclaw gets angry and frustrated, so she attacks again. The fight goes on for the whole day, but Mouthclaw can't get close enough to Sunpelt to kill him. She becomes too weak to fight, and promises to Sunpelt that if he will spare her life, she will grant him one wish.
Sunpelt thinks about what he could wish for. He then chooses and tells Mouthclaw that he wishes that she will be the length of a cat's tail, then she could do no harm to the Clans. But Mouthclaw tricks Sunpelt into bringing a thousand snakes the size of a cat's tail to the forest, all deadly and fierce, but all not as dangerous as Mouthclaw. Sunpelt feels guilty for what he did, as Mouthclaw outwitted him. Goldenstar scolds him, but then he tells Sunpelt that each snake is not as deadly as Mouthclaw. The snakes continue to reproduce, and live at Snakerocks until the Twolegs destroy their territory.
- "She was the only snake in the entire forest. She had killed many great warriors from all the Clans. Her sharp-fanged jaws could swallow a living cat whole, and she spat deadly venom."
- —Narrator describing Mouthclaw Secrets of the Clans, page 162
- "Mouthclaw slithered out of her cave, her tongue flickering like lightning. She was ten fox-lengths long and as thick as a badger with a bellyful of cubs. Her eyes were evil red slits, and her scales glittered in the dawn light."
- —Narrator describing Mouthclaw as she leaves her cave Secrets of the Clans, page 162
- "She bared her fangs with pleasure, for LionClan warriors were one of her favourite meals, and then she lunged."
- —Narrator describing Mouthclaw's hunger for battle with Sunpelt Secrets of the Clans, page 162