Skeletons of fallen Heroes and long ago explorers. Their remains may hold unknown wealth or better weapons.
There are different variations of skeletons, which can allude to what it could potentially be holding. Each dungeon drops weapons a tier lower than it and all weapons from the dungeons previous to it, so the Golem Dungeon drops Tier 0 weapons, while the Tech Dungeon drops Tier 0-3 weapons. The Wanderer Dungeon does not drop weapons; instead, it drops different tiers of potions depending on what floor you're on.