The Tentacles seem to be in reality just an appendage of a mass of Thing flesh that can stick to walls. This fleshy mass attacks by releasing a single green tentacle appendage that lashes at its victims and retreats back into the creature when no disturbance is detected.
Tentacles are not very intimidating foes, and can be dealt with swiftly by shooting quickly at them with any weapon. The player should be warned though, as they can deal significant damage if standing still too close to them, but they are easily dispatched. The Tentacles won't attack if the player is too far away.