Need Monster Help [OG]

I have the party going through the underdark and freed enslaved people. As they travel along to get them to topside, I want it to feel like a horror film where a monster takes away one person at a time until they notice which will make them panic.

So I need a list of monsters to pick from. Invisible stalker, ghosts, and skulk are a few that come to mind, but I am hoping for more options.