Although mainly noted for his comedic episodes, Vince Gilligan's intention with Roadrunners (2000) was to create "an all-out scary, creepy, get-under-your-skin - literally and figuratively - X-File."
The slug creature was created as a functioning remote-controlled animatronic, rather than CGI. The working model proved to be so realistic that producer Paul Rabwin noted that some of the cameramen were "starting to lose it" during filming.
This is the only episode that Vince Gilligan wrote in Season 8 because he was heavily involved in the production of The Lone Gunmen (2001) at the time.
The parasitic creature was designed to look like a banana slug.