A Marisa whose accessory is a kotatsu.
The kotatsu helps keep them warm during winters though it doesn't seem to cause them to overheat during non-winters. Sometimes shown that they can "grow" stuff you normally see on kotatsus after eating (such as mikan or tea).
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.
To differentiate kotatsu marisas from regular marisas using a kotatsu, a kotatsu marisa would not have the witch hat that a regular marisa has.
This tag implicates marisa (learn more).