Here is the full list of all 20 Guano achievements.
You got hit by a boatruck multiple times
You defeated Kevin!
You bullied Shy Gary for Seagullini
You pooped on a human for the first time
You opened the game!
You hit the same donut billboard as in the intro
You pooped on Seagullini. That'll teach him
You helped Dr. Squawkson test her directional buoys.
You helped Dr. Squawkson test her bouncy buoys.
You delivered the "secret ingredient" to Kevin
You defeated Kevin, again!
You delivered the "secret ingredient" to Dunga.
You destroyed Food Inc. buildings for Krakatoa.
You started a boatruck chain explosion
You farted 3 times while your poop was reloading
You pooped 25 times
You super screamed at a human
You unlocked all the game's concept arts!
You defeated Evil Guano.
You released a nuclear poop on Pohemeda Islands