A 'Trapper' is a small creature (roughly the size of a small dog) with the ability to teleport targets to a bleak "Trapper Dimension". They are blind, but can hear very well. If they detect the player, they will teleport them to the Trapper Dimension, and the player must go through a series of obstacles to return to reality. The Trapper itself does not seem able to teleport, and will die after causing another being or object to teleport.
The player may sneak by or used a ranged weapon to kill the Trapper and avoid teleporting.
Like most enemies, Trappers are aligned with one of the three main Ancients. This does not seem to have an effect on the Trapper itself (besides appearance), but determines the alignment of enemies encountered in the Trapper Dimension.
Oddly enough, despite being blind, Trappers can investigate and apparently see things while being controlled by players. The autopsy page for the Trapper also depicts it as having a large, fanged mouth, even though no mouth can be seen on the in-game Trappers.
Towards the end of the game, in the ruined city of Ehn'gha, one of the rooms Alexandra Roivas gets teleported into are filled with invisible Trappers, that you must use reveal invisible on them to kill or avoid them without "touching" them and being sent to the "Trapper Dimension". These are the only invisible Trappers in the game.