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.

One individual Seafolk is relatively powerful on its own, showing a threat level of "Wolf." 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.

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..."