monsters hit with an illusion effect
 

when a mob is hit with a spell like Ignite Bones that does a skelly illusion and damage when the illusion wears off the mob either has a different head or model the originally it had.

Examples:

LDC in solb is red with black legs, after illusion it takes on the brown model like the ro spiders.

Kobolds re-emerge with a bat head

Fire Giants are shrunk and with a cyclops style body.

nightmares in fear will turn white ... maybe because they got so scared when theyr flesh got ripped of theyr bones =O

Leave this one alone imo, its fun ;).