X or Ex Sackett, son of Gilbert Sackett and Sabra Jane Cook, was born in Michigan about 1858.
He was listed in the book Ancestors and Descendants of Frederick Plummer Sackett as X. Sackett. Few records have been found. None suggests a more conventional alternative name and it would seem that he was indeed named X (or Ex).
No explanation for this unusual given name is found and it would appear that X’s parents alighted on the name some 160 years before Elon Musk reinvented it.