Here is the full list of all 20 Pony Island achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Learned every secret of your past.
Found the hidden profile screen.
Escaped the arcade.
Destroyed the third Core File.
Found all 24 tickets.
Killed Louey.
Learned a third secret of your past.
Destroyed the second Core File.
Found Missingno.
Learned another secret of your past.
Found an alternate avatar.
Found 14 tickets.
Unlocked Pony Wings.
Found 7 tickets.
Found 3 tickets.
Destroyed the first Core File.
Learned a secret of your past.
Unlocked Pony Lasers.
Crashed the game.
Found your first ticket.