This reference is the absolute most notable one, while at the same time, it isn’t the most obvious.
Kapar can be found mainly Kirby’s Dreamland 3 and occasionally in Kirby 64. It was an enemy that gave Kirby the cutter ability, if Kirby so decided to inhale it. Its enemy behaviour was that it pulled off the sharp crown it always wore and threw it in Kirby’s direction. Until now, KRR has mainly referred to Kapar as a type of duck (as given away by the beak). The main connector between Kapar and his mythological partner is that Kapar is very adaptable in aquatic places and is usually only found near or in water.
Kapar is an offshoot of Kappa, however, that was probably given away by the name itself. Kappa are water imps that originate from the Shinto religion, that points them to be one of the many Suijin (water kami). They are often depicted as a sort of humanoid, monkey like posture, skin similar to that of a frog or lizard, a duck’s beak, a turtle shell, and they smell like fish. Their heads are bowl-like and retain water that ‘helps’ them to breathe while on land and not in water. If the water was to spill, a Kappa becomes immobilized and risks a chance of getting sick or dying if they don’t run back to water really fast. This “head water” is also where Kappa get most of their fighting strength.
Evil-hearted Kappa are bad news. They steal crops and kidnap children (among other things I don’t think I am able to mention). Evil Kappa also love to eat young children, though they will eat adults as well if they so decide. If they manage pull a human underwater, they will feed on them by sucking them dry of their mystical Shirikodama (Blood, “live force”, or vital organs, depending on the legend). It is rather disgusting how they go about doing that as well, of course, I'm sure we can all agree death by having each organ of yours ripped out one by one can be the utmost painful.
Based on this, it is safe to say evil Kappa were used a folk tale or "scare tactic" used by parents, meant to scare troublesome, rude or stubborn children into behaving rationally. This serves many uses, it keeps the children from wandering too close to bodies of water, where they may drown if they fell in and cannot swim out. This made some sense, since during the time in history myths of evil Kappa emerged; drowning deaths of children were very common. The other use was to teach manners. Even the most aggressive of Kappa are said to have a high sense of etiquette as a sort of weakness. If you bow to a Kappa, they cannot help but to try and return it, thus the water they retain in their heads will spill and they will be weakened. Children are taught to be courteous and bow as a defence against Kappa. The only other weakness a Kappa can have is that they are afraid of fire. Firework festivals are held each year at certain villages to ward off aggressive Kappa and other evil spirits. However, over the years, these unsavoury legends have been put aside, preferring the much more pleasant kinds.
Non-aggressive Kappa are quite nice, preferring cucumbers over human Shirikodama. In fact, they downright love cucumbers. They can be curious of human kind and some are known to speak Japanese. They also directly encounter people are challenge them to tests of skill like a sumo match or chess-like games for entertainment. They also will help farmers irrigate their land or share their knowledge of medicine and healing if a human becomes friends with a Kappa. Kappa cities were also heard of, in small underwater caves located close to the water. These cities have everything human towns have, like Restaurants, Hotels, Hot Springs, and Entertainment/Art Centers. However, Kappa still carry quite a good sense of manners and tradition. If a Kappa was to start saying snide remarks at you, all you need to do is bow, and the Kappa will be compelled to bow back, making his head water spill so he will have to run back to water. It’s almost comical.
There have been many Kappa sightings all over Japan over the past few centuries, almost in the same form as Nessie (the Loch Ness Monster) or Bigfoot. Because of this, Kappa have a few pages written about them in Crypto-zoology documents and books.