From the experts:
Rollo is a livelier, roly-poly, o-ending version of Roland.
Although it sounds modern, Rollo actually was seen regularly on Latin documents in the Middle Ages, though not heard in everyday speech. Historically, it's associated with a powerful Viking leader--who had a wife named Poppa.
Rollo May was a well known existential psychologist. There have been Rollos on screen played by Buster Keaton ("The Navigator"), Rainn Wilson ("Juno") and John Cleese ("Fierce Creatures").
Rollo has also been used over the years as a pet form of Roland, Rudolph and Ralph.