From the Guardian:
"Otters are not trainable," [Gaiman] explains. "Dogs are trainable - if you want them to sit you train them and give them rewards and they sit each time. But otters... if they do something cool and you give them a fish, the next time they'll do something even cooler. Or they'll try to do something completely different. I think that most writers - or at least a lot of us - are otters."