Skipper

Skippers are melee mobs that spawn in the Swamp Biome of the Overworld. It is found in water pools. It is a Hunter mob that can only be damaged when the player is level 5 or higher. This mob can't be damaged by fire, lava, or drowning and takes up a 1x1x1 space.

References/Sources
http://adventofascension.gamepedia.com/Skipper