Where did the name of Mr. Tumnus, the helpful faun of The Lion, the Witch, and the Wardrobe, come from? Gallic chieftains, of course! Where -umnus and -umnos were frequent components, as in names Togodumnos and Dumnorix. Of course, these were also latinised; the only way we know these names today is through Roman chronicles.
The -us ending of Tumnus is clearer: in Latin, it denotes a male name. The meaning of Tumn- is up for grabs however. I’m sure Lewis did not intend it as British slang for the stomach area, so I propose it derives from Autumn, being as the character functions as a bright spot of Autumn left in a wintry and frozen landscape.
Looking at the Chronicles as a whole, I am also pretty sure Lewis may have thought of The Western Wild of Narnia as having Celtish or Gallic-derived inhabitants. The lion skin that starts all the trouble in The Last Battle was said to have come from a human hunter far upstream of The Great River; tellingly, Lewis refuses to tell the reader his story. Earlier, in The Voyage of the Dawn Treader, Caspian and crew stop at a deserted island where they find “ruins of stone huts… also some bones and broken weapons” which screams Bronze Age to me, as well as the small coracle (helpfully explained as a boat “made of hide stretched over a wicker framework”) Reepicheep appropriates. In rereading this passage, I’ve noted they don’t spare any tears for the former inhabitants. In Lewis’s childhood, barely out of the Victorian Age, more primitive peoples were thought of as subhumans, models to be discarded on the way to higher civilization.
Making up a Gallic chieftain of your own? Here are some names… and if you need a tribe, it’s here.