It turns out "mad hatter" is more than the name of a weird rabbit. Hat makers had the disease named after them. Hats used to be made using the skin of animals, before synthetic fabrics were invented, and workers had to soak the animal fur in urine to soften it. But then, they switched to mercury, which regularly exposed workers to gas. It caused them to develop tremors, their teeth to fall out and other neurological problems. Hence the term, the mad hatter.