Named both for an old Irish farming tool and a piece of piping used in early toilets, spud wrenches have a tapered, spiky tail on one end. The head mechanism be anything from a regular, fixed wrench to an adjustable grip or specialty fixture. The name comes from the spike part of the tool, which can be used to help line up holes for bolting and welding (or to harvest potatoes).
Its use as a standard hardware element is credited to Thomas Maddock (1818-1899), an English inventor whose devices contributed to the advances of indoor plumbing in the late 20th and early 21st century. He sold his "Maddock spud" wrench design to the Standard toilet company, which used it until roughly 1920.