Green’s Dictionary of Slang

(US) a Greyhound bus.

1979		Hiler Monkey Mt. 160: Always the bus. Jesus, I got tired of ridin’ the Dog [HDAS].
1996		Eble Sl. and Sociability 81: The Hound and the Dog for Greyhound bus are associated with African Americans and students, though they originated among truckers.
2000		V. Hendricks Voluntary Madness 73: I’ve ridden the dog before, once all the way to California and back.
2007		N. McCall Them (2008) 93: He [...] trudged five miles to the bus station and caught the gray dog to Atlanta.
2013		J. Stahl Happy Mutant Baby Pills 52: Riding the Dog. When’s the last time you traveled by long-distance bus? Or sat in a Greyhound station?
2017		T. Pluck Bad Boy Boogie [ebook] ‘You don’t got a driver’s license, no picture ID [...] So it’s the grey dog, but it beats walking’.