BossBar

Added in version 2.4
Requires Minecraft 1.9 or later

The BossBar feature is used to give your custom bosses a healthbar in the same style used by the Ender Dragon and Wither plus a bunch of customization options.

internal_mobname:
  Type: <mobtype>
  BossBar:
    Enabled: [true/false]
    Title: '[name]'
    Range: [range]
    Color: [color]
    Style: [style]
    CreateFog: [true/false]
    DarkenSky: [true/false]
    PlayMusic: [true/false]

• Available colors for Color (case sensitive):
  • PINK, BLUE, RED, GREEN, YELLOW, PURPLE, WHITE

• Available styles for Style (case sensitive):
  • SOLID, SEGMENTED_6, SEGMENTED_10, SEGMENTED_12, SEGMENTED_20

• Range is the distance the BossBar will be displayed to players around the mob

• CreateFog, DarkenSky, and PlayMusic are buggy and do not currently work as of 2.5.0

Test:

Mobtype: creeper
Display: '&cTest'
Health: 20
BossBar:
  Enabled: true
  Title: 'Test'
  Range: 20
  Color: RED
  Style: SOLID

http://imgur.com/4aY6oUD