Term used to describe the last turd in a pooping session. More specifically, the turd that you didn't know was coming until you stand up and begin to fasten your pants, much like the slowest cow in a moving herd.
Joe: Dude, are you done in the bathroom? We're gonna be late!
Tom: Yeah, I'm comin'! I thought I was done, but I had a behind-the-herd turd.
n. A collection of many smaller sized turds, generally resultant from drinking too many cheap beers, such that after a bowel movement, the bottom of the bowl resembles a still of bison moving across the plains.
Wow - I had way to many Golden Anniversary's last night. Right after breakfast I dropped quite a turd herd.