Monster

[Kedjane]

Thinking about it Frictional actually made some great choices with the enemies in Penumbra:
Spiders are generally creepy and can make a great scare even though you pretty much only have to step on them in order to kill them, which makes a great "early enemy".
Dogs are great both for extremely intense moments where you have to hide from them while they slowly search for you, aswell as heart-pumping chases once you've been detected.
Rock worms are large, extremely powerful and make a great "hidden menace". They cause the walls to rumble just to let you know they are there and when they finally show themselves all you can do is run like the wind and hope for the best.
I also voted for zombies and humans. One of them could make a difficult fight later in the game, or possibly try to trap you or something like that through scripted sequences.