Muckdweller

Neutral mobs that spawn in swamps and mangrove swamps. They are amphibious, bipedal creatures a block and a half tall with a spinney dorsal fin on their back. They come in brown, green, and rarely in sky blue. Muckdwellers carry stone axes to hunt tropical fish and fight slimes. Maybe they could use spears as a new weapon, as weaker versions of the trident. They swim fast but are sluggish on land. Being small, they're not exactly strong, but a group of them can be trouble. On death they drop their axe and tropical fish. 

In normal swamps they rarely spawn, but in mangroves they thrive. They could have small settlements with huts of mud and leaves with a campfire in the middle of the colony. Maybe they could have a trading system like that of piglins. Their currency could be slime related, so they trade for slimeballs, slime blocks, and magma cream.