Spore Skeleton

Nothing summons it. It spawns naturally, but only down at cavern depth and only inside a real glowing mushroom biome, meaning a proper spread of mushroom tiles rather than a token patch. It will also use bare glowing mushroom blocks as a spawn surface, not just the grass. That combination is why a small planted plot never produces one, and why the surface version of the biome gives you spore zombies instead.

Mechanically this is a plain skeleton wearing fungus. It moves the same, it fights the same, and it hands over the same loot, so there is nothing exclusive to farm here. The Spore Bat sharing the same caves is the one with a reason to hunt it. Treat this as background traffic while you are down there for mushrooms or for the bat.