When is a horse a motor vehicle?

In the State of Texas, a motorist commits the offense of driving while intoxicated if he operates a motor vehicle in a public place.

Motor vehicle is defined in the Texas Penal Code as "a device in, on or by which a person or property is or may be transported or drawn on a highway, except a device used exclusively on stationary rails or tracks."

Recently in Austin, two men were arrested for DWI when police discovered they were riding a horse and a mule down Sixth Street (perfectly legal in Austin) while drunk. While it is pretty clear from the video that at least one of the men is not, shall we say, in complete control of his faculties, did they commit a criminal offense?

While the definition of motor vehicle in Texas is quite broad in scope, a horse or a mule is not "a device." There's also the issue of whether or not a motor vehicle must have a motor. To claim that a horse is a motor vehicle is absurd given the principle that every word in a statute means something.

So, the Austin Police Department charged two individuals with a crime they didn't commit and that the state can never prove, just to get some publicity. It would seem that if anyone is charged with a criminal offense it should be the police officers who should be charged with official oppression for subjecting an individual to an arrest that the officer knew was unlawful.