The Houndeye is a creature that appears in the first Half-Life game and its various expansions. One of the many forms of Xen wildlife, these bizarre alien animals were first encountered by Dr. Gordon Freeman during the Black Mesa Incident.
The Houndeye's body consists only of a thorax, two front legs and one hind leg. Its body is covered in wrinkled yellow skin with a set of electric-blue stripes running over its back. Where a head supposedly should be is a massive compound eye similar to that of a fly, though the Houndeye's eye is protected by an eyelid.
Houndeyes earn their name by the distinct sounds they make. They emit high-pitched barking sounds like Earth dogs and make similar movements as well, although more awkwardly since they only have one hind leg.
The Houndeye is an excellent example of a social animal in terms of pack hunting. While a shy and timid creature by itself, groups of three or more display resonant behavior, emitting destructive harmonic sonic attacks in shock waves, capable of injuring those they attack or destroying nearby objects such as wooden crates or windows. The shock waves are always visible. The eyelids are seen moving only when they are excited.
Although they are not seen displaying this trait often, Houndeyes appear to be carnivorous creatures, as one is seen eating a dead security guard in Half-Life: Blue Shift. The Houndeye's mouth is located on the underside of its belly, close to the eye, filled with what appear to be fang-like incisors.
It appears that, the more Houndeyes there are in a single squad, the more dangerous their shock waves are. A lone Houndeye won't do too much, but a pack of at least 4 can really do some damage. If a squad leader is killed, the entire squad is weakened. Their shock wave colours range from white, sky blue, dark blue, and violet, white signifying a weak shock wave.