For previous Halloweens, I've basically chosen Silas' costume for him. But this year, he knew what he wanted to be months in advance: a hermit crab. So, I got to work, made felted claws which are really just elongated and pointier mittens, and then crafted a hat with googly eyes. There wasn't really a lot on Ravelry or the Internet to go on in terms of patterns, particularly for the hat, so I made it up as I went along. I had all the yarn in my stash already (brown sheep lambs pride), so it was also a low cost costume.
The costume was a joint effort with my husband. He took a bike helmet (same one we used to turn Silas into a turtle his first Halloween) , and used papier mache to sculpt it into the shell you see below. This last picture was taken weeks before Halloween, when I had just finished his mitts and hat. He was very excited to be a hermit crab and would grab my hand and gently pinch it. "See mommy, I'm a nice crab!" Yes indeed.