For a first seamless pattern I put some skulls and bones together. 

For pattern brushes I choose some stroke patterns to simulate hatching, hair, fur or even feathers.

I used the brushes for the illustration below. Here I also created a stamp brush for the wheel rims and a texture brush for the house wall in the background.

I am working on a flamingo pattern to be printed. So I tried some of the brushes from above and added some waves.