Fishmen are not so much a race as they are an unfortunate side effect of the Evil Eye's Rune Cannon. A direct blast of the Evil Eye on a human exposes them to another dimension, thus altering their form into a grotesque fishman. In this form they are generally incapable of speech or any form of self control, though this is not always true. The condition is unfortunately irreversible, despite studies by the sorcerer, Simeon to find a remedy.
One of the earliest known instances of fishmen occurred before Kooluk's southern expansion into the Island Nations. Walter, his son, Kyril, and his servants had been searching for information on the Rune Cannons and ran into these fishmen on more than one occasion. Their search eventually led them to the pirate Steele, who had gained an Evil Eye. He used the weapon against Walter, which transformed him into a fishman and ultimately led to his death.
Following the war with Kooluk, the fishmen began to resurface after a crafty Kooluk merchant named Iskas was able to gather an Evil Eye cannon for himself in Warlock's old research area in Middleport. With no regard for life, Iskas used the Evil Eye to transform anyone he could into fishmen as loyal, unquestioning minions. His methods and plans soon were exposed leading to several Kooluk royals and soldiers turning against him. Iskas eventually turned himself into an unusually powerful fishman as a final act of spite against Kyril and his companions. With the defeat of Iskas, the Evil Eye was destroyed, presumably ending the existence of fishmen along with it.