If you lounge around in your bathrobe watching TV and ordering out for pizza, you'll get called a sloth. A sloth is actually a slow-moving, tree-dwelling mammal, but it has become a synonym for 'lazybones.'
Slug also doubles as the name of an animal and a term for someone who is lazy, slow or lethargic. But with slug, the animal name probably morphed into the human application, whereas sloth has meant "lazy person" or "laziness" for a long time. In the Catholic Church sloth was categorized as one of the seven deadly sins.