If fed properly, your pet will sometimes poop, and a poo will appear in your Pakka's room. To clean it, drag the poo into the trash bin. There is a chance the poo turns into a Poo Item that can be used for alchemy. These are all the obtainable Poos.

Mango poo
Mango Poo
Happy poo
Happy Poo
Blueberry poo
Blueberry Poo
Monster poop
Monster Poo
Fertilizer poo
Fertilizer Poo
Ghost poo
Ghost Poo
Kitty poo
Kitty Poo
Golden poo
Golden Poo
Angel Poo

An easy way to get your pet to poop is to feed them certain items like an Alien Meal that makes them poop with each bite.

Sometimes, when you open up the main room there will be a poo with a fairy flying around sparkling. If you tap the fairy or throw out the poo, you'll get 1000 hearts.