You may want to display your mobs health to players who are fighting it, there are a few different ways of doing this and this guide will go over them!

Boss Bars

You can create a Boss Bar for your mob and use it to display your mobs health much like the Ender Dragon or Wither has. The process is simple.

SkeletonKing:
  Type: WITHER_SKELETON
  BossBar:
    Enabled: true
    Title: '&cSkeleton King!'
    Range: 50
    Color: GREEN
    Style: SEGMENTED_12

Nameplate

You can display the mobs health in its nameplate above it, you can do this many different ways, here are 2 examples, one which shows the mobs health as a number (15/20) and another which has a bar that changes as the mob loses health.

Number

You will need to use the SetName mechanic and the ~onDamaged trigger. We will use the mobs display option to set the information. We are using the caster.hp and caster.mhp placeholders to get the current HP and the max HP.

SkeletonKing:
  Type: WITHER_SKELETON
  Display: '&6Skeleton King &7(<caster.hp>/<caster.mhp>)'
  Skills:
  - setname{n=<caster.name>;delay=2} @self ~onDamaged

Health Bar

This is a more complicated process but the same basic premise. In this basic example we will setup a Metaskill which sets the mobs name depending on the mobs health using health modifiers, and we are setting the starting bar when the mob spawns

SkeletonKing:
  Type: WITHER_SKELETON
  Skills:
  - skill{s=HealthBar} @self ~onDamaged
  - setname{n="&8[&a||||||||||&8]"} @self ~onSpawn

HealthBar:
  Skills:
  - setname{n="&8[&a||||||||||&8]"} @self =91%-100%
  - setname{n="&8[&a|||||||||&7|&8]"} @self =81%-90%
  - setname{n="&8[&a||||||||&7||&8]"} @self =71%-80%
  - setname{n="&8[&a|||||||&7|||&8]"} @self =61%-70%
  - setname{n="&8[&a||||||&7||||&8]"} @self =51%-60%
  - setname{n="&8[&a|||||&7|||||&8]"} @self =41%-50%
  - setname{n="&8[&a||||&7||||||&8]"} @self =31%-40%
  - setname{n="&8[&a|||&7|||||||&8]"} @self =21%-30%
  - setname{n="&8[&a||&7||||||||&8]"} @self =11%-20%
  - setname{n="&8[&a|&7|||||||||&8]"} @self =0%-10%