A covering for a koyukkuri's mamumamu and anyaru. Younger koyukkuris generally don't have bladder/bowel control yet and often uncontrollably piss and shit everywhere (especially when they're sleeping) so the goal of a diaper is to at least prevent them from making a mess everywhere.
It has been recommended that owners should not use comfortable, expensive products as diapers as it could cause the yukkuris to think shitting themselves whenever and wherever they want is a good thing. A more acceptable method is using an uncomfortable object (such as a lacquer tree leaf), which will encourage yukkuri to using toilet as a more desirable option.
It can also be used as a punishment and/or abuse device as seen here:
Often used by wild_yukkuris in the form of a leaf.