|The following is based on Van Buren and has not been confirmed by canon sources.|
Giant leeches or radioactive leeches range from around 2 feet long to the size of a full grown pot-bellied pig. These critters are very slow and will attack on sight.
The Hoover Dam Scum Pits are occupied by giant leeches that swim in and out of the large holes in the ground where the power generators used to be and are big enough to suck out enough blood to kill a super mutant. They also attack the giant albino rats in the Scum Pits.