Alien Leech

Leeches as seen in Half-Life 2.

Leeches are alien creatures used underwater - in Half-Life and its expansions they are killable, while in Half-Life 2 they are used to stop players from swimming out of maps and are unkillable. Whether these are from Xen or Earth is not known.

In Half-Life, Leeches were more of an annoyance than a threat, although the player could still be killed by them. However, in Half-Life 2 the leeches are much more dangerous, and a few bites will easily result in the death of the player. In Opposing Force, they are observed in the Hydrofauna labs of Black Mesa, which means that they could be an alien, though this is not yet proven.

Interesting to note is the absense of Leeches in the teleporter sequence of Red Letter Day, replaced with a fleeting glimpse of a hungry Icythosaur.