An experimental form of arthropod-based bio-weapon created by Umbrella co-founder Dr. James Marcus, the Plague Crawler was part of his early trials with a newly created variant of the Progenitor virus known as "t". They were created by manipulating the genes of various insects with the virus.
They were considered unfit for heavy combat due to their lack of intelligence and control, and so were subsequently disposed of. However, a few of the specimens survived and managed to procreate outside unnoticed, even as far as Raccoon City itself. Several of the surviving Plague Crawlers were later encountered by S.T.A.R.S. Bravo Team recruit Rebecca Chambers and ex-lieutenant Billy Coen during their exploration around the site.