I came across this washed up fish in Panama City, Florida in November 2015. I'm guessing it's a puffer fish but I can't find anything like it online.
This is a kind of trunkfish. (They have different names, this could be a smooth or spotted trunkfish.). It's really a lovely and comical little fish when observed alive in coral reefs. It has the ability to change its coloration depending on whether it's excited or calm, or to minimize its contrast to the background. It is related to puffer fish.
It has a boxy, triangular body shape, and propels itself with relatively tiny, delicate fins. Like pufferfish, they are toxin producers.
In death, the body shape and coloration are different, of course. Never saw a dead one before; sad. The juveniles are adorable:
Members of this family occur in a variety of different colors, and are notable for the hexagonal or "honeycomb" patterns on their skin. - Wikipedia