A groundhog has been making weather predictions in Punxsutawney, Pennsylvania, since 1886, but we've only been calling him "Phil" since 1961. Before that, the critter was usually just called the "Br'er Groundhog" or "The Punxsutawney Groundhog." Most sources (including the Punxsutawney Groundhog Club) say he eventually earned the moniker "Phil" as homage to "King Philip," but that explanation is as dubious as it is vague.
The problem is that they never specify which King Philip. The tradition of foretelling the weather with a marmot's shadow has its origins in Germany, but Deutschland hasn't seen a "King Philip" for more than eight centuries. France, Greece, Spain, and even the Wampanoag people of New England have all had a King Philip, but it's very unlikely that a small Germanic Pennsylvania community would ever name their beloved groundhog after any of these kings, either.
Rather, the name might actually refer to a prince—and it may have gotten its start thanks a pair of heinous murders and some good old-fashioned small-town competition.
In 1953, Punxsutawney sent two baby groundhogs to Los Angeles's Griffith Park Zoo. The critters had been named after Britain's new reigning couple, Queen Elizabeth II and her husband, the future Prince (not King!) Philip. While the zoo gladly welcomed Liz and Phil with open arms, the state of California did not. The California Department of Agriculture declared the baby groundhogs "agricultural pests" and demanded they be "destroyed." The animals were summarily killed.
Back in Pennsylvania, people were deeply insulted. (The Punxsutawney Groundhog Club went so far to say that the groundhogs had been "executed.") The head of the Punxsutawney Groundhog Club, who worried that the killings could spark an international incident, told the Los Angeles Times, "I'm going to ask [my congressman] to take the matter up with the State Department so we won't get into complications with England. Killing these groundhogs was an insult to the royal family." Indeed, a congressional representative would issue a statement criticizing California. The two groundhogs were eventually buried back home.
Eight years later, the name "Punxsutawney Phil" first appeared in newspapers. It's possible that this new moniker was a shout-out to one of the dearly deceased royal groundhogs. (That, however, is a matter of speculation.)
Regardless, the new name was also a necessity. Multiple Pennsylvania towns—such as Quarryville and Pine Grove—also had their own prognosticating woodchucks, and the towns were stuck in a vicious debate over who was home to the real sage. Adopting a new name was not only good branding, but also a practical way to help differentiate the different groundhogs. (The competitors would also get unique names: Octoraro Orphie and Grover.)
Eventually, Punxsutawney would get a huge PR boost from the 1993 movie Groundhog Day—though it was always home to the leading marmot. On Groundhog Day in 1904, the Pittsburgh Press reported, "The ticket-sellers in the various railroad offices noticed a surprising increase in receipts this morning. First-class rates to Punxsutawney went so fast that the advisability of raising the price was considered. All the cold weather interests were off to the lair of the groundhog to see him see his shadow."