Black shark eyes
Scales (onyx, iridescent, crystalline, metallic, etc.)
Razor-sharp claws
Bony protrusions
Multiple eyes
Lashing tail
Leathery wings
Spines
Dripping ichor
Glowing eyes or markings
Hair or fur (drifting as if underwater, burning with a cool fire, etc.)
Feathers
Multiple arms
Tentacles
Hard shell, metallic plates
Lights dim or flare
Plants wither or grow wildly
Mechanisms grind to a halt
Liquid freezes, boils, turns to blood or ashes