I found the Ichthyosaurs in Half-Life to be truly scary. I mean, they're pretty creepy to look at, but the main reason is that they're so dangerous. They move
fast. They make that...
sound (which I don't even know how to describe) when they're barrelling towards you to try to bite you in half. And you have to leave your comfort zone in order to fight them, because it's very difficult to kill the damn things from above, you have to get in there, in the murky water, where it has the advantage. Also, most of your weapons don't work underwater.
And then there was that part where you had to navigate the slippery, crumbling walkways above the pool occupied by two or three of them.
"
Don't slip, don't slip"
*slip*
OH GOD