Custom Mob Bestiary
The model is part of the mechanic. Armour, weapon, scale, glow and particles tell you what a custom mob is about to demand.
Silhouette
Large shields, heavy weapons and unusual height normally signal control or burst danger. Identify that before entering melee.
Particles and phases
A change in glow, particles or posture can mark an active ability or phase shift. Create distance until you recognise it.
Pack problems
If a model looks like a floating vanilla item, run /classicpack before judging the encounter.