(backstory) I had been doing some seed hunting and world exploring, and when I made a soul sand valley floating islands world, and checked my inventory, It had turned into the 2D sprite model for the cod and I found that almost nothing (joining other worlds, and putting on a texture pack) seemed to make a change. Even though closing and restarting the game seems to help, I'd still suggest looking into this odd glitch.

Edit: This glitch seems to randomly happen and has no source.