Looks like a big rotting fish corpse to me! Seagulls probably picked off the skin and scales, taking all of the yummy fat with it. The pharyngeal arches look like they’ve flipped up and out of position to form the ‘horns’ on the head, likely a result of this critter drying out in the hot sun.
Bet it smells lovely, hah!
BTW there was another ‘floating sea monster corpse’ story that cropped up recently. That one was a dead whale.
Forever seeking: Blackwatch the raffle Old Warrior, Jennifer Miller's pieces, and GB Baby unis!