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 all 24 tickets.
Found Missingno.
Learned a third secret of your past.
Found an alternate avatar.
Found the hidden profile screen.
Escaped the arcade.
Destroyed the third Core File.
Found 14 tickets.
Killed Louey.
Destroyed the second Core File.
Learned another secret of your past.
Unlocked Pony Wings.
Destroyed the first Core File.
Found 7 tickets.
Learned a secret of your past.
Unlocked Pony Lasers.
Crashed the game.
Found 3 tickets.
Found your first ticket.