The Viper is a very large boa constricter.
The Viper is first seen in Rubystar's den. She is talking to Shadowheart, when the Viper strikes. When Leopardspots hears the commotion, and runs in, the snake bites her. Along with the help of Troutleap and Troutstream, they manage to get the snake out of the camp.
The snake then travels to the ThunderClan camp, and attacks Firepelt there. Firepelt would have died, if it wern't for his extra lives. The snake slithers out, but promises to return.