Sneak (to move stealthily): sneak > sneaked > sneaked
The past tense and past participle of sneak are always sneaked. The nonstandard snuck sneaked into the language in the 1900s, growing in popularity in the 2000s. How it appeared is unknown—no other -eak or -eek word forms the past tense with -uck. While snuck has become more popular in American English, it remains nonstandard.