"Snuck" is the standard past tense and past participle of of the verb "to sneak" in American English, although in British English the past tense and past participle is actually "sneaked."
I am an American born in the late 20th century in a highly educated family and having moved in highly educated circles my whole life, but I had only ever heard, saw, or read "snuck" as far back as I can remember for my whole life. Thus I was stunned to learn recently that "sneaked" is actually the British form, and that it was the original form used in America as well until recently. I'd literally never heard the word before in almost three decades of life.
And it's not that I never read British books or talk to British people, because I do, but it probably has a lot to do with the fact that most of my encounters with British English are in an academic context, and "sneak" isn't exactly the most academic of words. Well, that and the fact that all of the British English was censored out of the American version of Harry Potter.
But it is also a testament to the astounding and complete success of the form "snuck" from its lowly origins as an example of backwater bumpkin speech to becoming the standard form for most English speakers on Earth, as well as one of the only known examples in English of the irregularization of a regular verb in recent times.
"Sneak" is a right good English verb, likely dating back to the Old English form snícan, and first appeared in its present form in (where else?) the works of William Shakespeare, such as "A poor, unminded outlaw, sneaking home" (Henry IV, part 1), or "Sneak not away, sir; for the friar and you must have a word" (Measure for Measure). But from Shakespeare's time up to the 19th century, both in Britain and everywhere else, sneak was a regular verb with a past tense and a past participle of "sneaked."
But somewhere in America around the mid 19th century, the past and participial form "snuck" emerged as a regional variant, and first began appearing in print, usually to mockingly represent uneducated speech, in the 1880s. For example an 1887 edition of the New Orleans Lantern quoted a country person as saying, "He grubbed ten dollars from de bums an den snuck home."
What makes this form so odd is that there are no other examples in English of a verb with a present tense form ending in "-eak" or "-eek" having a past or past participle form ending in "-uck."
But nevertheless, by the 1920s and 30s the form was starting to appear among educated speakers, although used only jocosely. From then on it gradually but steadily became more and more common among all speakers of American English.
Dictionaries generally held out though, declaring it "incorrect," "non-standard," or at best a "regional variant" well into the 1990s. As late as 1988 67 percent of a panel of American experts voted to declare "snuck" to be "incorrect usage."
But by that point the cat was already out of the bag, as far as snuck went. By the mid-1990s, "snuck" had surpassed sneaked in educated usage in America, and was used in almost 100 percent of cases by people under 30 years of age.
Today, you can still find lots of cases of people claiming that snuck is "wrong" or "non-standard" but this is absolutely false. It is now the standard form across all strata of American society, including newspapers and academic writing, and is also widespread in Canada and Australia. A Google search of "snuck" vs. "sneaked" reveals that at least on the internet, "snuck" blows "sneaked" out of the water, more than doubling its number of hits, 2,480,000 to 1,160,000 (the difference is even more staggering on Everything2, with 2430 instances of "snuck" and only 572 instances of "sneaked"). And with almost all young people today using snuck and only Britain holding out, its continued ascendence going forward seems assured.