Denis
This is the French form of Dionysius, the Greek god of wine and revelry.
Derek
From the Old German, 'ruler of the people'.
Dermot
From the Irish 'Diamid', meaning a 'free man'.
Dexter
From the Latin, meaning 'skilful'.
Digby
From this surname, and means 'one who lived by the dyke'.
Dirk
A variant of 'Derrick'.
Donald
From the Celtic, 'proud ruler'.
Dorian
From the Greek, meaning 'one who came from the ancient Greek place named. Doria'.
Dougal
A Scottish form of 'Douglas'.
Duncan
A Scottish name, meaning 'black haired'.