Seafolk

Seafolk（海人族, Kaijin-zoku / 深海族, Shinkai-zoku) are a race of beings that have come from the sea to take the surface of the earth for themselves.

Overview
Seafolk are a violent race who wish to take the surface of the Earth for their own and turn the human population into their cattle. They are large humanoid creatures that have various aquatic animal features, such as gills, tentacles, fins and other sea creature-like traits.

Even the messenger of the Seafolk is a threat level of "Wolf." Combat Seafolk have been shown powerful enough to lift a truck with one arm. Several of them together is even more of a threat, coming in at a level of "Tiger". A group of Seafolk has been shown to be able to push an A-Class hero to his limits.

Sea Monster Arc
The first Seafolk rises from the sea and starts to head inland, but before he could even leave the water, he is destroyed by Saitama. Some time later a large wave hits the shore of J-City carrying several Seafolk with it. The Seafolk then rise from the water and begin to wreak havoc in the city. The hero Stinger then shows up and saves a citizen from the grip of one of the Seafolk.

Later the Seafolk are shown on a news report to have worn down the hero Stinger to his limits, but are defeated.

The Sisters Who Have Too Many Things Happening
During the Seafolk invasion, a shrimp-like member of the species is backed into a corner by the Blizzard Group, Lily tries to go in for the kill, only for the Seaflok to attack her with Acid Spit. Fubuki appears and disperses the Acid Spit before it can hit Lily and then proceeds to kill the Seafolk.

Webcomic and Manga Differences
In the manga, Seafolk attacked by the shore, in the webcomic it was in the city.

Quotes

 * (As one is killed by Saitama) "Then, you humans shall live on as our cattl-GROAAAAAAAAAAAAHHHHH..."