Evil Dragon Adopt Me. It was released during the 2019 halloween event, from 18 october 2019 to 1 november 2019. Now it is unavailable so players only can get it by trading.
If you don't have enough money, another player can help by giving you their shadow dragon, which allows you to cast a shadow of them on the map. Now it is unavailable so players only can get it by trading. The bat dragon, along with the evil unicorn, was an exclusive pet released during the 2019 adopt me halloween.