Just something I thought of when I saw the Elder Guardians. They sure look like ghostly like paintings with the eyes that follow you when you walk around. They do tend to flap around alot so you can pour water to calm them down in doing so it would cause their spikes to retract slightly more as they can can be quite annoying.

Command:
/summon Guardian ~ ~ ~{Elder:1,NoAI:1,Silent:1b,Invulnerable:­1}