The Fishmen are one of the native sentient species of Third Earth, who resembled bipedal fish-like creatures ( All the fish families based of Earth fish ).The majority take appearence of the Ostariophysi based fish ( Suchs Carps, Loachs, Goldfish, Minnows, Suckers, and Sucking Loaches )
A group of their kind lived in a paradise oasis, somewhere in the Sand Sea; until their home was attacked by the massive Ramlak, which consumed all the water of their home. With their home destroyed, Captain Koinelius Tunar, became fixated on getting revenge on the creature; and assembled a crew of Fishmen along with a ship to hunt it down. Their journey saw them encounter the ThunderCats, under Lion-O who they captured, and initially attempted to flay as well as eat until the Ramlak attacked.
Upon seeing the courage of Lion-O, Captain Tunar decided to let the ThunderCats live and asked for their help in slaying the beast. Ultimately, the Fishmen's ship was destroyed by the Ramlak and Captain Tunar was killed but Lion-O managed to slay the creature leading to all the captured water being released into the Sand Sea thus creating an oasis for the Fishmen. The Fishmen thanked Lion-O and decided to remain behind at their new home.