Potamoi

The Potamoi were a group of exclusively-male freshwater gods who appeared in much of the mythology of Ancient Greece.

Overview
Said to number three thousand (a figure which was synonymous in classical Greece with "countless"), the Potamoi were the children of the Titan siblings Oceanus and Tethys, and brothers to the Oceanids. Gods of the many rivers and streams of the world, they took one of three forms: the first that of a beautiful human male, the second a bull with the head of a man, and the third a merman-like creature with the tail of a fish, torso of a man and the head of a bull.

History
Several of the Potamoi took part in major events in Greek mythology such as Scamander, who fought by the side of the Trojans during the Trojan War and nearly succeeded in killing the mighty hero Achilles, and Achelous, who was bested by the demigod Heracles in a wrestling competition for the hand of a mortal woman.