A dog. Obscure origines possibly deriving from the propensity of dogs to cheerfully eat their own excrement.
"Joe-Bob's old turd hound came a-loping out from under the porch with this shit-eating grin on his face."
An otherwise known smart dog breed, but this particular one is embarrassingly dumb. Eats feces and rocks. Often can be heard barking at nothing. May also be a normally beautiful breed of dog, but has ugly aesthetic features such as; wire coat, beard, Grinch-like feet.
Jim's dog is a Border Collie mixed with turd hound, pretty sure I saw him hunting for a shit snack in the yard this morning.