It replaced a Fakemon called "Hydrop" that was in the game before the full release.
It is based off a Kappa, an amphibious river yōkai demon or imp found in traditional Japanese folklore. They are typically depicted as green, human-like beings with webbed hands and feet and a turtle-like carapace on their backs. The dex entry mentions about people giving it cucumbers, referencing that Kappas are said to have a love for cucumbers.
The name comes from Kappa (a creature of East Asian mythology) and Splash.