Hunting Spiders.

[BrittanyPanthas]

Spawn in stealth, but don't attack anything, even once attacked they remain passive until they're dead.  Even saw NPCs attacking one like this.

[Tankerman]

I can confirm this, re: "don't attack anything".

[athousandyearsofpain]

I've seen this happen once too. NPC attacking it and it doesn't fight back.

[FleetingHeart]

Let me know if this is still happening.

[General-shadow]

Nope
meet a hunting spider came up behinde me and slit my pretty lil throat wide open

[Lulzebub]

Oh God, I wish they had used stealth in the UD. That would have made them ten times more scary than they already were.