The Atlantean Fish is an enemy unused in the first Tomb Raider game. It very much resembles a catfish, as it possesses long whiskers. Due to it's removal, fish enemies would not make their debut until the following game.
Like other fish enemies in the Tomb Raider series, these also merely try to bite Lara.
The level in which the fish was found within suggests that it would have appeared in Atlantis.