Hidden Jerry

Hidden Jerry is an enemy that has a chance of spawning when mining, logging, killing mobs, and harvesting crops, but only when Mayor Jerry is in office.

There are 4 types of hidden Jerrys; green, blue, purple, and gold, each having 3, 4, 5, 6 health respectively, but can only take 1 damage per hit, though burning effects can surpass the defense. When killed, they drop a Jerry Box that corresponds to the color (gold Jerry drops gold Jerry Box)

When a Hidden Jerry is found, a message will appear in your chat letting you know, and a floating head will fly around for about 3 second before spawning a Hidden Jerry. Once spawned, the Jerry will quickly run away, sometimes falling into the void or off a high area, causing you to lose the box (if you hit the Jerry first, then the box will drop if it dies from fall damage). Using a bow from a distance can, however, be used to shoot the Jerry from a distance without it running away. The Jerry spawns randomly but has a minimum cooldown of about 6 minutes

For more details, see the Jerry Box wiki page.