The Xen Ichthyosaur is an aquatic animal originating from the alien realm of Xen in the Half-Life game series. These creatures were prominently featured in Half-Life and its expansions, and it also made a cameo appearance in Half-Life 2.
Although the in-game creature is named after the extinct marine reptile group, the Ichthyosaurs, it resembles them only in its carnivorous ecology and its general size. When the Ichthyosaur is first encountered in the Half-Life chapter Apprehension, a scientist remarks that while he has been told it was hauled from the Challenger Deep in the Pacific Ocean, he believes that the creature never swam in terrestrial waters until a week before. This is possibly the time of its arrival at the Black Mesa Research Facility, the creature being likely from Xen. However, there is evidence that Ichthyosaur specimens were kept for observation, as seen in Opposing Force.
In Half-Life, Ichthyosaurs are encountered in several water bodies, including a flooded laboratory, the reservoir near the hydroelectric dam, the sewage areas, and one of the Nihilanth's chambers. They are often placid at first, but quickly become hostile, especially should the player take action against them. The speed and viciousness of the Ichthyosaur, combined with poor underwater visibility, limited choice of weapons and limited oxygen while underwater, make the underwater areas of Half-Life some of the most frightening parts of the game.
The Ichthyosaur also makes a single appearance in Half-Life 2. During the teleport malfunction ("harmonic reflux"), Gordon Freeman is momentarily transported outside of City 17, in midair. He looks down, and falls into the ocean. Underwater, he is almost swallowed whole by an Ichthyosaur before being transported elsewhere. Although this encounter is scripted, when an Ichthyosaur is spawned using console commands or a mod such as Garry's Mod it also attacks the player in the same manner.