Here’s the costume I made for my son a year ago.
Am a slooooow poster. Am posting just in time for this halloween. As a starting point I re-used a fleece pyjama I had made for his sister, but you could use any shop bought one. I added a hood with felt teeth and eyes, and added a strip down the back which ended as a tail, all studded with points. Then I cut out disks in a contrasting colour and sewed them on randomly.
Not sure if he’s a monster or a dragon, anyhow, he loved his looks.
I made the costumee a month early, silly me. He grew so much I had to add a gusset between legs, and new feet 😛