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1.
Slang name for the North American Pronghorn (Antilocapra americana). So named due to it's close resemblance to a goat and it's incredible speed.

While it's difficult to measure their speed, most sources state upwards of 70 Km/h, and they're known to be the second fastest land animal, second only to the cheetah, but able to sustain higher speeds for longer distances than the cheetah.
Oh, Crap! We're going to hit that speed goat. Duck! Why do they hang out near the highway?

I've given up hunting speed goats because they taste like a cross between a Slim Jim and sagebrush.
by Sigmo January 02, 2011
 
2.
Noun: The poor, hapless driver who has been pulled over by traffic cops for speeding on a highway or country road. The 'speedgoat', through their sacrifice, allows all other drivers to speed past with impunity and without paying the ultimate price.

While passing, with a sigh of relief, we thank the highway gods that we are not the 'speedgoat'. But inevitably, that day and that siren comes for us all.

From the pig latin: scapegoat
Mark roared with glee as he zoomed down the highway, "Did you catch the look on that speedgoat's face?"

"Better watch it, sunshine," replied white-knuckled Jane from the passenger seat. "The way you're driving, your time will come!"
by Hipsterion February 28, 2013
 
3.
A goat of infinite speed. Often seen wearing a sombrero and poncho.

Speed Goats are close friends with Tai from Gears of War 2. Together they share Locust BBQ's.
"Dude, My nacho is gone!"

"Must have been a Speed Goat."
by Tres_Etrange July 11, 2009
 
4.
Any animal that willingly and suicidally places itself between your automobile and your destination at an inopportune moment. From the combination of Speed Bump + Goat (generic animal).
Watch out for those Speed Goats in the middle of Wyoming, they'll get you every time.
by CodeMan Sr. December 13, 2010