Prince Harry seems to have started a trend ... because it appears his grandfather just gave the world a royal wiener flash of his own.

91-year-old Prince Philip was hangin' out in a kilt during the Gathering for the Highland Games in Scotland this weekend ... when he decided to unleash his one-eyed Loch Ness Monster on the masses.

Philip was sitting near his wife, Queen Elizabeth, during the event -- the first time P.P. has been seen in public since suffering a bladder infection a few weeks ago.

God Save the Queen.