My 3.5 year old son insisted on being a Hammer Head Shark; a regular shark would not do, it had to be a Hammer Head. Naturally, we searched for examples but found very little.
I started with a grey hoodie sweatsuit and created fins from foam and grey felt. The head is grey felt stuffed with batting and the eyes are white and black felt. He did manage to whack many of his fellow revelers on the head whenever he turned, but the effect worked and I was pleased with the result, even though he decided to take it off halfway through the trick or treating.
The only thing I would have done differently would have been to sew more form into the head, so that it was not so wobbly.