I've used store bought patterns in the past & still do occasionally but many of the things for my 2 dogs have come from inside my head.
I've got lots of patterns I've made myself using newspaper. Usually I draw out what I've envisioned just to "see" it. It may take several tries but who cares?...the newspaper would've wound up in the bin anyhow!
Good luck & be sure to come back & show us what you've created.