Burnt Gyarados (also called Zombie Gyarados) is an easter egg skin of Gyarados. It is the evolutionary form of Burnt Magikarp and can only obtained by giving a Burnt Magikarp a Rare Candy at, or past level, 19. If leveled through training, however, it will evolve into the gyarados original skin.As of version 2.5.7 of Pixelmon, while Burnt Magikarp can still be obtained, Zombie Gyarados will revert to the normal Gyarados skin upon Magikarp's evolution. The Burnt skin can be seen briefly, but is reset when sent out.
However, a new way to get a Zombie Gyarados is to get a Burnt Magikarp. The previous way to do this was to set it on fire, but now you have to strike the Magikarp with lightning (/summon LightningBolt). This will make a burnt magikarp. Getting it to level 20 will cause the burnt magikarp to evolve into a burnt gyarados. The picture under the old version of the Burnt gyarados or Zombie Gyarados is a new version of the zombie gyarados.