Landfish

Landfish (Japanese Name) is a monster that was shown in the spin-off manga series known as, The Ways of a Monster Nation. It is known to reside in Tempest inside a dome which Rimuru made to mimic a forest.

Appearance
A fish that have 2 arms and a small fin on both sides, as well as 2 legs which it uses to walk on land.

Trivia

 * Although no real name was given, the nickname Landfish was made due to it being a fish which has legs.