Do guinea pigs like to sleep in an igloo?

If you mean the type of plastic igloo that you buy in pet-shops to put in cages then , yes they do. Guinea pigs love to have a small private place to sleep at night. You don't have to buy one of these though, guinea pigs would be just as happy with a cardboard box that they could chew on , you'd have to change it every week , or else it would get soggy and disgusting.