Story related, unmissable.
Obtained during Chapter 12: Where Crawls The Lizard? after defeating Nattie.
Nattie is the underwater piranha monster that you fight in the water tank in the final level of the game. As the battle starts, use to drain the water tank by pulling one of the wheels on the wall. This stops Nattie from diving underwater and attacking you. Pummel him with punches like you would any other boss, dodging his smashes with if your spider-sense flares up.
As you fight him, Nattie may call his Pygos friends, who look like white versions of himself. These are more of a nuisance than anything else. You can defeat them, but Nattie will resurrect them eventually, so it might be easier just to focus on Nattie himself. Keep the water level low with the wheels on the wall, and Nattie will fall to your fists soon enough.