The Cardiff Giant is a phenomenon only in the sense that its manufacture and fame were part of one of the greatest hoaxes in U.S. history. It was so famous that P.T. Barnum, the Circus promoter, had a model made when he could not purchase the original which he put on display in his shows. One of the producers of the original, who refused Barnum's request to purchase it, is reportedly the first person to say "there's a sucker born every minute," although the remark is often attributed to Barnum himself.
The original Cardiff Giant was manufactured by George Hull of New York, an athiest who had engaged in a spirited argument with a group of fundamentalist Christians during a church revival over the interpretation of Genesis 6:4
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Although geologists and archaeologists immediately declared the "giant" to be a fraud, many fundamentalist Christians insisted it was authentic, as it appeared to verify the comment in Genesis. In the end, it proved to be quite profitable for those who participated in its exhibition. So, it was a phenomenal fraud, but nothing more.