Whiscash is a dark blue, catfish-like Pokémon with a yellow underbelly. It has a cavernous mouth with light blue lips that have short, thin whiskers growing from the lower lips. Long, yellow barbels grow from its cheeks, and there is a yellow marking on its forehead that resembles a W. The light blue dorsal fin has a pattern of three spots arranged in a triangle, while its horizontal tail fin lacks spots. Its pectoral fins and belly fins match its main body color.
Whiscash uses its caudal fin to cause earthquakes that can extend for over three miles by hitting the bottom of swamp beds. Whiscash can predict when natural earthquakes will occur. It also uses these massive earthquakes to drive away any foes that come near it, going on rampages in the process. This behavior, along with sightings of Whiscash's leaping from lakes, has caused people to mistaken it for starting earthquakes. It lives in a nest at the bottom of ponds and swamps, and is territorial over them. Whiscash tends to wait silently for prey to come to it. This gluttonous fish will eat anything that is alive, sometimes whole, and has been shown in the anime to eat inanimate objects as well. Whiscash also has a diet of consuming mud. Whiscash has poor eyesight, so it has to use its whiskers to detect a prey's vibration.
Whiscash debuted in Whiscash and Ash. A fisherman named Sullivan had been hunting one nicknamed Nero for fifty years because of its huge size. After Ash had battled Nero and retrieved his Badge Case that it had eaten, Sullivan used it as his chance to finally catch Nero. However, even after using a Master Ball, Nero simply swallowed the Ball and swam away.
Pokémon Pinball: Ruby & Sapphire: After hitting the button changing the Pokémon bumpers on the Ruby Field a set number of times, Whiscash will appear in the pool. Hitting the ball into Whiscash's mouth will transport the player to the Sealeo Bonus Field. In the unlikely event that the ball hits Whiscash from behind, it will get angry and start an earthquake.
Whiscash is based on Namazu, a catfish in Japanese mythology that causes earthquakes (also note Whiscash's Japanese name "Namazun"). Its move Future Sight and ability Anticipation also alludes to this myth, which possibly originated from the observation that catfish are capable of predicting earthquakes and become agitated a few hours before their occurrence. Sometimes Namazu is depicted as dragon-serpent which may explain why it can learn the move Dragon Dance. The yellow zigzag on Whiscash's head is similar to a crown. Originally, Namazu served as the inspiration for Gyarados due to it's Beta Sprite but was later rehashed for Whiscash's design. Making both Whiscash and Milotic Pokémon based on Gyarados' design in some respects.
Since its pre-evolved form appears to be based on a loach, it may also draw inspiration from loach catfish. Whiscash could also be based on Electric catfish due to being able to learn the move Spark.
Whiscash may be a combination of whisker, mustache, and possibly catfish. The first letter of its English name may also come from the "W" shaped symbol on its head.
Namazun may be derived from 鯰 namazu (catfish) or 大鯰 Ōnamazu, a catfish in Japanese mythology which caused earthquakes.