Raincoat Zombie

It only appears on the surface at night while rain is falling, which makes it common without ever being something you plan around. Behavior is plain zombie: it walks at you, works around obstacles in its way, deals damage by touching you, and can only open doors during a Blood Moon. There is no ranged attack, and unlike ordinary zombies it has no armed variant hiding in Expert, so a raincoat is always exactly what it looks like. It comes in three sizes that differ only slightly.

The yellow coat is the useful part. In a night crowd it is the one silhouette you can pick out instantly, which lets you prioritize it if you want the Rain Coat and Rain Hat it wears. Those come off rarely enough that a dedicated trip is wasted effort. Rain is common and these things are harmless, so whatever surface arena you already built for Blood Moons is the right place to pick them up in passing.