Xavier is a Basque name meaning "new house," but for most American pet owners the anchor is Professor Charles Xavier of the X-Men: telepathic, composed, morally serious. It's a big name to hand a puppy, and that's precisely the fun of it.
The X-Men Anchor
Charles Xavier has been a fixture of superhero culture since the 1960s comics and gained new visibility through Patrick Stewart's film portrayal. Pet owners who choose Xavier are usually invoking that reference deliberately. Professor occupies similar ironic-dignified territory at even lower frequency.
The Human Name Crossover
Xavier is a consistently used human name in the US, which places it in the category of names that make a dog feel like a person rather than a label. Owners who want that quality find it fully delivered here.
The Counter-Reading: Resists Every Nickname
Xavier has no natural shortening: Xav isn't quite there, and X is more of a joke. Most owners end up saying the full three syllables every time, which is either a feature or a minor inconvenience. Deciding in advance which it will be saves some frustration later at the dog park.