Giant Leech

Not to be confused with Queen Leech.

The Giant Leech was created by accidental means when a t-Virus outbreak occurred in Raccoon City; the water supply was contaminated by Dr. William Birkin which very quickly spread the virus throughout the city. When the contaminated water reached the sewer system, the leeches which were already there began to change and mutate into more bloodthirsty and cunning creatures.

One such Leech grew to an incredible size, and had the ability to asexually reproduce and constantly supply more pre-infected leeches. The creature was so large that it was largely immobile, essentially a birthing pod.

While the Giant Leech is aggressive and has various methods of attack such as spitting acid and biting, it is more interested in performing its main function - keep making more leeches. As such, when encountered it will usually stop the attack if far enough away.

It's worth pointing out that like all leeches, it is very vulnerable to fire. The Giant Leech was ultimately destroyed in this way when a fuel canister was ignited and exploded.