How does a Pokémon learn an unreachable move?

//ads -- adsterra.com -- native banner
?>
For example, I see that Aegislash has the move Head Smash in its move list, but it actually can't learn it by leveling and both pre evolutions don't even have it. So how can it learn that move?
Pictures about "How does a Pokémon learn an unreachable move?"



Can India annex Pakistan Occupied Kashmir ( POK )? History and Geography of Kashmir
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Budgeron Bach, Budgeron Bach, Alex Green, Alex Green