Leech

Leeches are aquatic creatures from Xen that hunt in packs in oceans and tear apart any non-leech that enters the ocean.

Half-Life and Opposing Force
Leeches make their first appearance in Half-Life in some of the flooded sections of the Black Mesa complex, wherein they are called Hagworms. More information is revealed in Half-Life: Opposing Force. In addition to being teleported into Black Mesa by the Resonance cascade, they have were kept in the biodome and aquatic lab sectors for study. While moving through these sectors, Adrian Shephard killed most of the leeches that were kept for study. In both games, leeches are yellowish and can be killed easily with a single shot or a hit from any melee weapon. Additionally, when stranded out of water leeches flop around as most fish do.

Half-Life 2
During the Portal storms leeches were teleported to Earth and infested the earths oceans and they seem to have endangered or caused the extinction of most terrestrial sea creatures. They attack Gordon Freeman in a swarm (an attack similar to that of the pirahna) if he ventures too far into the ocean. The Ichthyosaur was originally intended to fulfill this role, but this idea was dropped for an unknown reason and leeches were used instead.

Minor differences
In Half-life 1, the leeches were relatively annoying, but never anything more than that, although in Half-life 2 they can drain all of your health in a matter of seconds. This may be due to the fact that in Half-life 2 they are in a more natural habitat (the sea). In Half-life 2, they feed off of the fishes in the ocean, however in Half-life 1, theere was no clear food source for them, which may have caused them to be cannibalistic at some times.

Leeches as food
In Half-Life 2: Lost Coast it is revealed that, since ocean fish are now very rare (or extinct), the human occupants of coastal areas catch and eat leeches instead. It is highly probable that leeches now act as a common food source to humans and vortigaunts.