Gruesome mystery (warning: graphic photo)

[Dash]

Petey the valentini was pecking at something stuck on the powerhead this morning and my stomach sunk thinking it was one of my fish. Quick glance at anemone, nope. Head count...all fish good, hermits yep. Flick on the light and the pistol shrimp immediately opened up his burrow. Hmm. So I examine this body and it's clear to me it wasn't a fish. I've cooked enough fresh seafood to conclude it was some kind of shrimp. The "head" has the stumps of all the little feet shrimp/lobsters have. But it can't be a molt - the whole body is attached and it's sizable - 1.5" without the shell.

This body, or even a molt, does not correspond to anything I put into the tank. Could a mantis or other shrimp have been hiding in my rock all this time? No shell found. Maybe it was molting and got caught by a puffer?
So weird.