Here, I used Hero Forge to depict Heracles (better known by his Roman name, Hercules) and the river god Achelous, ready to fight. I designed Achelous as an (anthropomorphic) hybrid of a bull and a snake. Achelous' coloring is said to be blue-green because of the symbolism of the river.

Mythology (as I know it)
In Greek mythology, Achelous is the personification of the river Achelous.
Achelous is considered the oldest and most distinguished of the Greek river gods. He is the son of Oceanus and Tethys. He is able to change his shape (an allusion to the river, whose deity he was, and which also repeatedly changed its course). As a river god, he is responsible for the abundance of fresh water and the fertility of the land.
When Achelous fell in love with Deianeira, daughter of Oeneus (and sister of Meleager), he came into conflict with his rival Heracles (son of Zeus), who intended to remarry after the tragedy that had befallen his first family. Achelous first transformed into a serpent; finally, into a bull, but this did not help him: Heracles broke off one of Achelous's horns while he was in his bull form. To retrieve his horn, Achelous had to exchange it for a horn from the goat Amaltheia, which Zeus had nursed (later called the cornucopia).
Heracles married Deianeira, and they had a son, Hyllus.