The Happy Ghost's shadow is glitched.

Using a boat as an example:

Steps:

1. Spawn a Happy Ghost.

2. Trap the Happy Ghost with blocks.

3. Make it fly at a height where its shadow is visible on the ground (not too high).

4. Tie a boat or other mob below it.

5. The circular shadow on the ground will show unknown outlines that rotate with your perspective.

Note: This occurs on both mobile (Android) and PC (Windows) versions.