ItemsAdder

This balloons type require ItemsAdder to work.

Creating a balloon

Step 1

Create an ItemsAdder entity reading the tutorial:

CreationItemsAdder Wiki

Step 2

Create the cosmetic configuration

  # Example custom kite created with ItemsAdder entities 
  # (supports also animated ItemsAdder entities models)
  star_kite:
    display_name: "Star Kite"
    type: BALLOON_ENTITY
    balloon:
      leash: true
    # Showing a different item into the GUI 
    # (in this case it's a must because ItemsAdder entities can't be put in GUIs)
    model:
      gui: minecraft:egg
      normal: cosmetics:star_kite
    dye:
      enabled: true # To allow this item from being colored.

As you can see I set the normal model to the name of the custom entity created with ItemsAdder.

Step 3

You're done

Final notes

You can even create animated balloons, just animate the idle animation of the entity using Blockbench. For example you can create a complex dragon or a rotating UFO.