From the Super Mario Wiki
Fishmael is seen in Paper Mario fishing by the docks at the port and arguing with a Fuzzipede. Parakarry and Mario can deliver Letters between him and his fishing buddies, Goompapa, Muss T., and Koover. He loves to fish and is always seen fishing in the ports of Toad Town. It is completely unknown what creature he is, although the fishing rod sticking out of his head resembles the antenna of an Anglerfish.