Mae dafad ddu Dafydd yn dod o De Ddyfed ond dydy dafad dew Dafydd ddim yn dod o De Ddyfed.
(”Dafydd’s black sheep comes from South Dyfed, but Dafydd’s fat sheep does not come from South Dyfed.”)
Via Wicipedia:Blwch tywod, where you can also find scandalous information about some guy named Carlo and a corgi. (Due to the sandboxiness of the link, all of the above may very well be gone by the time you get there.)