You guys are pretty new to this eh???
Screamer-Killers (not Scream Killers) are what they used to called the Carnifex's. If you look at the old model for it, it had eight scythe shaped arms and it looked like it was yelling. Hence the name. It could produce a plasma ball capable of doing tank like damage with the scream.
Hunter Slayers are termigaunts. The gaunts with the guns.
They weren't Eldar names for the species. Everyone, particularly Space Marines and Imperal Guard used to call them by those names. GW totally changed a bunch of stuff when they released the 3rd edition Codex. They did a good job on it too..