In Legends, the ancient Dark Side Dragon of the Hssiss species was a dangerous one. They eventually adopted the Dark Side that possessed them and over time were moulded by it. Just after Order 66, they actually were sent to find remaining Jedi and turn them to the Dark Side through a single bite or scratch. The rest of their information can bee learned about in this video. I wonder if they'll make an appearance in Star Wars The Last Jedi.

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