Leech (Resident Evil Zero)

Image of Leech
The product of Dr. Marcus' experiments were these giant leeches, and just like regular leeches they have a strong lust for blood. Dr. Marcus used them in his research to create the T-Virus, and it is these leeches that infected the passengers on board the Ecliptic Express, turning them into Zombie.

The sheer number of these leeches you encounter at once means it would take a very long time trying to dispose of all of them. It is therefore much easier to simply run through any area inhabited by them. In some cases the leeches will merge together to form a humanoid shape of their creator described as Mimicry Marcus.