The Sweeper was a B.O.W. (Bio-Organic Weapon) designed as an offshoot of the
Hunter II - a similar creature with more or less the same body type. Like most types of Hunter it was created by splicing human and reptile DNA and using the
t-Virus as a way to bind the two into a living entity.
The key changes are in the hue of its skin which is more red than green like most of the reptile-based Hunter designs which gives them a rather devilish look. They could also afflict anything they attack with a highly toxic poison which has the potential to kill a human in mere moments.
They were intended to be used in an area after had been attacked by other B.O.W.s, killing off anything that survived - essentially making a last sweep through, which is how they got their name.
A unique variant which may have mutated further due to a fault with the embryo existed which made it many times larger and more aggressive than a normal Sweeper.
It's also worth mentioning that the red Hunter which appeared in the
Spencer Mansion was not a Sweeper but was in fact an early prototype of a Hunter Alpha which was a much older model — only one of these was ever thought to exist unlike the Sweeper which was produced en masse.
