Icthyosaur
CONCEPT
The Ichthyosaur is one of many alien species encountered in Half-Life that have been teleported to Earth from the borderworld Xen


Appearances

This Xen (Location)-native creature was a common enemy for Gordon Freeman (Character) in water-logged areas in Half-Life. The first encounter with it is some point mid-game in a pool below a precariously placed diver's cage holding the crossbow after which it can be used to kill said Ichthyosaur.  It only appears a few times but appropriately at moments tense enough to give players a good scare when caught off guard by it or frantically swimming away from it.  The Ichthyosaur also has a slight cameo in Half-Life 2. While Gordon is being teleported around in the malfunctioning teleportation device at Dr. Kleiner's Lab, he warps in and out of space between random locations, one of them being some body of water containing an Ichthyosaur, which is slowly swimming towards Gordon and is just seconds from attacking him when Gordon warps back out.