Introduced in Dreadfall Event, Hybrid Dragons are the result of two different Dragon Species breeding and hatching an offspring that mixes both parents' traits into one. This results in the creation of a whole new Species with its own appearance and Stats.
While the How To Train Your Dragon franchise introduced the Hybrid Dragons with the Night Lights (Light Fury and Night Fury Hybrid), the first introduction to Hybrid Dragons in the game was during the Dreadfall Event with the Skrillknapper (Boneknapper and Skrill Hybrid) and the Dreadstrider (Flightmare and Speed Stinger Hybrid)