From the Super Mario Wiki
Fish Skeletons are joke items that appear in the Game Boy Advance version of Mario Bros. bundled with Mario & Luigi: Superstar Saga and the Super Mario Advance series. They only appear in the Mario Bros. Battle multiplayer mode as one of the items obtained from garbage cans. Fish Skeletons look like eaten fish with the head and tail intact that, according to the manual, smell funny. They are useless and can be thrown away.