The monsters in general. Being chased by them makes it even scary.
The Brute (pictured) is a massive golem that looks like that is made out from cooled lava, his loud footsteps are your indicators that it is near, oh and it's also the fastest among the monsters despite its large size.
The Hunter is a translucent slimy thingy that has long arms with talons, it has the tendency to ambush you.
The Fiend, is much different than the others because of its demonic appearance and its ability to levitate and possessing telekinetic powers, the way it kills you however, is more gruesome than the other monsters
One specific death from the Fiend involves him crushing your heart using his telekinesis.
If the Fiend catches the PC from behind, he uses his telekinesis to make the PC's head turn 180 degrees
Special mention goes to the Fiend's life-raft ending, which has it giving a really close Nightmare Face while the screen flashes.
You only face down one monster in a given playthrough, but the notes and audio logs make it clear that all three of them were out and about at one point (the Brute frying an engineer and melting a hole in the engine, the Fiend causing glitches everywhere, the Hunter coming out of the vents to get at the ship's food). Two of them are implied by the notes to be former crewmates being treated for burns. So first off, where did the other one come from? Second, where the hell are the two you don't face? They're clearly not on board anymore. What if they disembarked while the ship was at port in Hong Kong and are loose in the city or commandeered lifeboats of their own and attacked other ships? Best case scenario is that they fought each other instead and you're stuck with the one that survived.