Pet food can be made in the Barn from the fruits of Atlantean trees. The barn is already on the island, but it needs to be restored.

There are three types of Atlantean trees: Atlantean Quince, Atlantean Almond and Ku'rana Tree. They can be bought in the Shop.

For the tree to bear fruit, it must be fertilized with special substances made in the Botanical Lab. The lab is on the island, but it needs to be restored.

You'll need Mineral Dust to make fertilizers for Atlantean trees. Mineral Dust can be obtained after harvesting something on your garden beds.

The amount of mineral dust obtained from one  garden bed depends on the crop.
You can get:
1x  for Flax
2x for Corn
3x for Sweet potatoes
4x for Pineapple