The swamp monster is a semi-hostile aquatic mob found entirely in swamp biomes. It uses a revolutionary skin, being completely invisible when stationary and simply causing the air around it to darken when moving. The main point of this is to render the mobs very hard to spot. They are completely non-hostile, until such a point as an unsuspecting player swims around it, at which point they will retreat a few paces, then inflict 1 heart of damage per half second to the player. There is no way of stopping it.
The easiest method of avoiding them, because attacking is useless, is to simply stay on land. If no water blocks are present, they will be destroyed. Therefore, they cannot step on land, and so will not attack you if you stay on land.
Swamp monsters will only spawn in swamp biomes, and will spawn in 1/1000 surface water blocks.
Swamp monsters are a three block tall and two blocks wide, They have fins and gills. They have webbed hands, and dark red eyes, like spiders. there hands are very large for swiming. They only have two large toes for increased movement in water.