Here are some miscellaneous quotes I found on the Internet:

Well, if you haven't got the nerve of a government mule!
kicking like a government mule
legs like a government mule
being beaten like he was a "government mule," as WWF ring announcer Jim Ross likes to say
He had the guts of a government mule to go into Tokyo bay
he might as well try to make an actor out of a government mule
I knew about as much about it as a government mule