Fish-Men

Fishmen are one of two species that dominate the seas (the other being the Merfolk). In terms of leadership, none are mentioned, however Fisher Tiger, Jinbei and Arlong were said to have the highest respect amongst their people.

Appearance
Appearance-wise, they are more "fish-like" than merfolk, looking like a cross between a man and a fish of the ocean such as an octopus, manta ray, or sawfish; however, they still have legs.

Biology
They have their gills near their necks which only work when they are in water. On land they can breathe with their lungs like humans do.

Even though Fishmen are part fish, they are still mammals and give birth to live young.

As was revealed during the Davy Back Fight arc with Big Pan Fishmen can also breed with Giants, producing a hybrid who has biological abilities of a Fishmen and a huge body of a Giant (but still smaller than a full one, as related by Robin), they are known as Wotan. Although never shown it's clear that Fishmen and humans can also breed the same way Merfolk and humans can breed.

Inter-species Relationships
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.

A stereotypical Fishman considers humans an inferior, weaker species. However, this does not apply to all Fishmen: for example, the pufferfish shipwright Tom cared and looked after two human children (Iceburg and Franky) and even protected them from the actions of Spandam and CP5, and even the Shichibukai, Jinbei, considers the Whitebeard Pirate, Portgas D. Ace an equal and holds the Whitebeard Pirates as a group in high regard because of Whitebeard protecting Fishman Island.

On the opposite end of the spectrum, human relationship to the Fishmen was defined with their definition of both them and merpeople as "fish" instead of "men". This lead to centuries of discrimination as the Fishmen fought to prove their worth to the world. Only two hundred years ago did the World Government began to bridge the gap, making an alliance with Fishman Island to remove the problems between the two sides. While attempting to fix the problem, centuries of bad relations remained: many humans continued to see Fishmen as freaks or, in the case around Sabaody Archipelago, slaves for the Tenryuubito. On the opposite end, certain sects of Fishmen attempted to create their own societies where they dominated over the humans, such as Arlong Park in East Blue.

Overall Strength
They are ten times as strong as normal humans from birth, and this difference is magnified underwater, where a human's power is cut in half (while Fishmen are completely unhindered, and perhaps even strengthened). Jinbei himself noted that on dry land he had little power to offer, which considering the strength that even a fishman as powerful as he has speaks volumes about the potential of any fishman in the water compared to being on dry land. At the hands of a powerful fishman, even a small splash of water becomes a leathal weapon.

Their homeland of Fishman Island is also where you learn Fishman Karate, a number of fishmen have displayed skill in this fighting style already within the storyline. This fighting style in particular has both land and sea based attacks, a style suited to the fishman's advantage in the water, but does not leave them completely unable to defend themselves on land when they have to. Like Giants they have yet to display the use of skills such as Haki and the need for a Devil Fruit ability seems pointless for their race leaving them mostly a physically reliant race like the giants. However, their water based abilities prove most helpful considering that in water Devil Fruit users are rendered powerless completely, the Devil Fruit Kage Kage no Mi in particular has little advantage at all against a Fishman who knows its fatal weakness.

Because of their strength, they appear in One Piece on a regular basis.

Translation and Dub Issues
The kanji for Fishmen (魚人) is the reverse of the name for Mermen (人魚). They are often incorrectly referred to by fans, and the English dub made by 4Kids, as Mermen for that reason.

Trivia

 * Originally Oda planned on introducing Fishmen as far back as Chapter 3. However, due to his editor he then put it back, ending up introducing the concept during the Arlong arc instead.