Madog Fychan ap Gruffudd Fychan

Since he does not appear to have been part of his brother's revolt, he was probably dead before 1399.

Darrell Wolcott, Center for the Study of Ancient Wales : " Numerous modern biographies, including Wikipedia, include a statement saying "Owain probably had an elder brother called Madog, but he may have died young". We suggest he didn't "die young", but eluded the detection of modern writers by being called "Madog Fychan" in the pedigree manuscripts":

Please see Darrell Wolcott: The Ancestry of Owain Glyndwr; http://www.ancientwalesstudies.org/id200.html. (Steven Ferry, September 13, 2020.)