When Hiccup and Toothless save the Zippleback's life, the grateful dragon abandons Ruff and Tuff, and follows Hiccup and Toothless trying to please them in any way possible.
When Hiccup and Toothless save Barf and Belch from a Snow Avalanche, the Dragon betrays The Twins for Hiccup. They try says to get the Dragon back to the twins but nothing works. When Hiccup gets taken, the Zippleback comes to Hiccups rescue.