Ned comes from
Ferdinand because
Ferdinand contains the letters N and D. I don't worry excessively about the E because vowels in English nns aren't necessarily correlated with the name. For example,
Jim and
Jem are nns for
James,
Jenny originated as a nn for
Jane,
Midge is a nn for
Margaret,
Millie can be a nn for
Amelia, etc.
Ned from
Ferdinand is a bit of a stretch, but as I've discovered, finding formal names for which
Ned can be a nn is extremely difficult. Sometimes you have to take what you can get.