I didn't find anything resembling the previous links, but you might be able to get something by going to Fish Pics at Great Lakes Fishery Commission and look for the Brown Bullhead, about half-way down the page. The Channel Catfish is just a couple below it, but is much too "pretty" to be mistaken for a "mud cat."
There are those who disagree, but there is no single "mudcat" accepted by any large group of people. Various local regions identify different critters by this name. Our founders may have had a particular regional type in mind; but in different areas you'll find different fish using that alias.
In Kansas, the "bullhead" is the one most commonly called a "mudcat," mostly because it looks like it should spend its time in the mud. It actually is found most often in comparatively clear water, and eats less "carrion" than many other catfish. It's one of the very few catfish that can often be caught with artificial lures since it frequents clearer water and hunts at least partially by sight. Most catfish hunt mainly by smell (or possibly by electronics).
A little southeast of here, the name is usually reserved for what my F&G people call the "madtom" catfish. You might try searching that name.
The most deserving of the name around here is probably the flathead, since it does (especially when large) spend a lot of time just "laying in the mud;" but they get large and make great sport to catch, so nobody uses the "derogatory" name for them much.