A Marisa whose accessory is a kotatsu. To differentiate kotatsu marisas from regular marisas using a kotatsu, a kotatsu marisa would not have the witch hat that a regular marisa has (as the kotatsu is an accessory and therefore replaces the hat).
The kotatsu helps keep them warm during winters though it doesn't seem to cause them to overheat during non-winters. Conversely however, kotatsu Marisas are sometimes shown to be weak against cold and will quickly die if they're removed from their kotatsu. Sometimes shown that they can "grow" stuff you normally see on kotatsus after eating (such as mikan or tea).
Adult kotatsu Marisas tend to have big enough kotatsus that they can share it with other yukkuris or even sometimes with humans.
Some stories explain that kotatsu marisa are a variant of shell marisa. Though unlike shell marisa which is mostly a birth defect, the kotatsus on marisas are largely beneficial. Just like shell marisas, while kotatsu marisas are somewhat rare they don't receive any special treatment from humans unlike other rare species and are abused just the same.