Is the word Snuck used as the past tense of the verb Sneak or a real word?
There is a helpful summary in The New Fowler's Modern English Usage by R.W. Burchfield (OUP 1998).
Sneak (verb) : Its origins are shrouded in mystery. From the beginning and still in standard British English, the past tense and past participle forms are sneaked. Just as mysteriously, in a little more than a century, a new past tense form snuck has crept and then rushed out of dialectal use in America, first into the areas of use that lexicographers label jocular or uneducated and more recently, has reached the point where it is a virtual rival of sneaked in many parts of the English-speaking world. But this is not in Britain where it is unmistakably taken to be a jocular or non-standard form.
Bryan A. Garner calls snuck nonstandard in his Dictionary of Modern American Usage (OUP 1998).
Some British dictionaries provide usage notes warning against the use of snuck.