Sampson is the variant spelling of Samson, the biblical figure of supernatural strength, famous hair, and a profoundly instructive mistake involving a woman and a pair of shears. On a large, powerful dog, the name carries exactly the right associations. The alternate spelling is probably a personalization choice rather than a distinct etymological form.
The Biblical Strength Reference
Samson/Sampson is one of several strength-associated names (alongside Thor, Hercules, Titan) that owners apply to large breeds as a kind of aspirational label. The biblical story adds depth: this isn't just strength, it's strength with vulnerability, which is actually a more interesting set of associations for a family dog than raw power alone. Rottweilers, German Shepherds, and Great Danes appear with this name most often in registry data.
The Spelling Note
Sampson with a "p" appears in historical records as an English surname form; it was used as a given name in colonial America and is documented in the human Sampson page. It's not a typo of Samson; it's a legitimate variant with its own history. That said, it will occasionally be corrected to Samson by people who consider their spelling the canonical one.
Counter-Read
Sampson works best when the dog actually has some physical presence to live up to. A Chihuahua named Sampson is a different kind of joke: intentional irony rather than sincere aspiration. Compare Samson for the standard spelling side by side.