In episode 22 of XYZ, Jimmy's Pikachu uses dig. According to Pokémon DB, Pikachu cannot learn dig by level up or TM/HM. Anyone got a reasonable answer?
Pikachu learns Dig by TM in Gens 3-6. Because Dig is no longer a TM as of Gen 7, the site's main page for Pikachu says it does not learn Dig by TM. However, the page for Pikachu's learnset in previous generations does include Dig as a move it can learn.