Fish-Men

Fishmen(魚人 Gyojin), often incorrectly referred to by fans, and the English dub made by 4Kids as Mermen, are one of two species that dominate the seas (the other being the Merfolk). Fishmen come from Fishman Island, an island in the Grand Line. Appearance-wise, they are more "fish-like" than merfolk, looking like a cross between a man and a fish of the ocean such as Octopus, Manta Ray, or Sawfish, yet they still have legs. They are twice as strong as a human on land, and ten times stronger underwater. A typical Fishman (although this does not apply to all Fishmen) considers humans an inferior, weaker species. Because of their strength they appear throughout the entire series of One Piece on a regular basis. Their race includes the pirate Arlong and his crew.

Even though fishmen are part fish they are still mammals and give birth to live young. Their view of life under the sea is dominated by their views that they should rule over all other fish in the sea. Despite this view, usually they get along well with the Merfolk who share the opposite view on life, however it is not unknown for Fishmen to take advantage of the Merfolk's nature. Known types of fishmen are sawfish, octopus, cutlassfish, manta ray, squid, swordfish, mudfish and most likely whaleshark.