Rollo is a Latinized form of the Old Norse Hrólfr, meaning "famous wolf," borne by the first ruler of Normandy in 911. The television series Vikings has since restored the name to contemporary visibility with a large, physically powerful character. For a big dog, the historical and pop culture associations are genuinely apt.
The Viking History
Rollo's historical life reads like mythology: a Norse raider who negotiated himself into the French nobility, whose descendants would include William the Conqueror. For a large, imposing dog, the association is perfectly apt. Great Danes, Norwegian Elkhounds, and Nordic breeds generally carry Rollo with the weight it deserves.
The TV Series Boost
The History Channel's Vikings (2013-2020) featured a Rollo character played by Clive Standen: large, loyal, ultimately significant. That portrayal has refreshed the name's pop culture availability for owners who aren't medieval historians.
The Counter-Reading: Sounds Round and Funny
Rollo has a slightly comic sound: two syllables, both round, bouncing off each other. On a physically impressive dog it resolves into dignity. On a small, rotund, waddling dog it becomes charming irony. Both outcomes are perfectly good outcomes for a name this flexible.