I bumped into these fella's in the Army warehouses as well as X18. I have not seen them in any other area's though, there nearly always accompanied by some snorks and sometimes the odd Psuedogiant if I'm really unlucky. Who am I kidding, If I'm lucky.
What their resistance? Stronger than bloodsuckers, weaker than chimeras?
They dont feel overpowered I think there close to a Chimera. I remember fighting one in the swamps in Narod Solyanka and it could eat 3-400 shots of buck and I only managed to kill it because I found out it was scared of water. Luckily this is not like that, It's no pussy though.
I am really looking forward to finding my first librarian, and killing it.. Do they act the same as in Metro? Man I loved that game and cannot wait for the sequel.